Hypothyroidism

Hypothyroidism is an endocrine disorder characterized by inadequate production or function of thyroid hormones, resulting in a decreased metabolic rate that affects virtually every organ system in the body. In farm animals, this condition most commonly results from dietary iodine deficiency, which prevents the thyroid gland from synthesizing adequate amounts of the essential thyroid hormones thyroxine (T4) and triiodothyronine (T3). The thyroid gland, located in the neck region adjacent to the trachea, requires iodine as a fundamental building block for hormone production, and when iodine is lacking, the gland enlarges in a compensatory attempt to capture more iodine from the bloodstream, resulting in the characteristic visible swelling known as goiter.

Hypothyroidism affects all farm animal species including cattle, sheep, goats, pigs, and poultry, with prevalence varying significantly based on geographical location and dietary management. Regions with iodine-deficient soils, including many inland and mountainous areas, historically had high rates of hypothyroidism and goiter in both livestock and humans before the widespread implementation of iodine supplementation programs. While modern mineral supplementation has dramatically reduced the incidence of iodine deficiency hypothyroidism in well-managed livestock operations, the condition still occurs when supplementation is inadequate, when animals consume goitrogenic plants that interfere with iodine utilization, or when iodine requirements are increased during pregnancy and lactation.

The economic and welfare impact of hypothyroidism in farm animals can be substantial, particularly when it affects reproduction and neonatal survival. Thyroid hormones are essential for normal fetal development, and maternal hypothyroidism during pregnancy can result in stillbirths, weak neonates with congenital goiter, or offspring with developmental abnormalities. In adult animals, hypothyroidism impairs reproductive efficiency, reduces growth rates, decreases milk production, and compromises overall productivity. Wool and hair coat quality may deteriorate, and affected animals are more susceptible to cold stress and infections. The cumulative effect on herd productivity can be significant even when individual animals show only subtle clinical signs.

With appropriate diagnosis and treatment, hypothyroidism in farm animals is highly manageable and often completely correctable. When iodine deficiency is the underlying cause, supplementation of the diet with iodine leads to resolution of clinical signs and restoration of normal thyroid function. Prevention through adequate trace mineral nutrition is straightforward and cost-effective, making hypothyroidism largely preventable in well-managed operations. Understanding the causes, recognizing the clinical signs, and implementing appropriate prevention and treatment strategies are essential components of comprehensive farm animal health management.

The primary cause of hypothyroidism in farm animals is dietary iodine deficiency, which occurs when animals are fed forages or grains grown on iodine-deficient soils without adequate iodine supplementation. Iodine is essential for the synthesis of thyroid hormones, and the thyroid gland has no alternative mechanisms for hormone production when iodine is lacking. The body has limited ability to store iodine, so continuous dietary intake is necessary to maintain normal thyroid function. Certain geographic regions, particularly inland areas, mountainous regions, and areas subjected to heavy rainfall and glaciation, have inherently low soil iodine content, and feeds produced in these areas reflect this deficiency. Without supplemental iodine, animals consuming diets based on these feeds will develop hypothyroidism.

Genetic and breed predisposition to hypothyroidism in farm animals is less well characterized than dietary causes, though some forms of thyroid dysfunction have hereditary components. Congenital dyshormonogenesis, a genetic defect in the enzymatic pathways required for thyroid hormone synthesis, has been documented in certain sheep and goat breeds and results in hypothyroidism and goiter despite adequate dietary iodine. Some degree of individual and family variation in iodine requirements and thyroid efficiency likely exists across breeds, though this is not well quantified. Inbreeding may increase the expression of recessive genes affecting thyroid function. Certain breeds used in intensive production systems may have higher iodine requirements due to greater metabolic demands.

Environmental and management factors strongly influence the risk of hypothyroidism, particularly those affecting iodine availability and requirements. Consumption of goitrogenic substances, which interfere with iodine uptake or thyroid hormone synthesis, is an important cause of hypothyroidism even when dietary iodine appears adequate. Goitrogens are found in various feedstuffs including brassica crops (cabbage, kale, turnips, rapeseed), soybeans, millet, and many other plants. These compounds include thiocyanates, isothiocyanates, and goitrin, which can block iodine uptake by the thyroid or interfere with hormone synthesis. Nitrates from fertilizers or certain plants also have goitrogenic effects. Additionally, certain environmental contaminants including perchlorates can interfere with thyroid function.

Risk factors for developing hypothyroidism include geographic location in iodine-deficient regions, reliance on locally produced feeds without iodine supplementation, and diets high in goitrogenic feedstuffs. Pregnancy and lactation significantly increase iodine requirements, making gestating and lactating females more susceptible to deficiency. Young, growing animals have higher requirements relative to body size. Winter feeding regimens relying heavily on stored forages from deficient areas pose higher risk than summer grazing of diverse pastures. Operations that do not provide mineral supplements or use supplements without adequate iodine content are at increased risk. Climate change affecting soil mineral dynamics and changing feed sources may alter risk patterns over time.

The pathophysiology of hypothyroidism begins with inadequate thyroid hormone production, which initiates a cascade of metabolic effects throughout the body. Low circulating levels of T4 and T3 are detected by the hypothalamus and pituitary gland, which respond by increasing production of thyroid-stimulating hormone (TSH) in an attempt to stimulate the thyroid gland to produce more hormone. This chronic TSH stimulation causes the thyroid gland to enlarge (goiter) as follicular cells proliferate, but without adequate iodine, increased thyroid mass cannot compensate for the hormone deficiency. The resulting low thyroid hormone levels cause widespread metabolic slowing, decreased oxygen consumption and heat production, slowed growth and development, impaired reproductive function, and reduced immune function among many other effects.

Early warning signs of hypothyroidism in farm animals are often subtle and nonspecific, making early detection challenging. Initial symptoms may include slightly reduced growth rates in young animals, subtle changes in hair or wool coat quality, and mild decreases in feed efficiency. Affected animals may appear somewhat sluggish or less active than normal herdmates, though these changes can be difficult to appreciate in group housing situations. Reproductive efficiency may decline slightly, with longer intervals between estrus cycles or modest reductions in conception rates. These early changes often progress slowly over weeks to months, allowing for significant thyroid dysfunction to develop before clinical signs become obvious enough to prompt investigation.

Common symptoms vary somewhat among species, though the core manifestations of reduced metabolic rate are consistent across farm animals. In cattle, goiter may be visible as a swelling in the neck region, though the degree of enlargement depends on the duration and severity of iodine deficiency. Affected cattle often have rough, dry hair coats that shed poorly and may develop myxedema, a characteristic puffy swelling of the skin particularly noticeable around the face. Milk production declines in dairy cattle. In sheep, wool quality deteriorates with fibers becoming weak, brittle, and prone to breaking; open fleeces allow increased cold sensitivity. Goats show similar signs with rough coats and decreased milk production. Pigs develop thick, dry skin and sparse hair coats. Poultry show decreased egg production and poor feather quality.

Behavioral changes associated with hypothyroidism primarily relate to the decreased metabolic rate and reduced energy levels characteristic of the condition. Affected animals typically appear dull, lethargic, and disinterested in their surroundings. They may be slow to rise and reluctant to move. Activity levels decrease, and affected individuals often seek out warm areas and show increased susceptibility to cold. Appetite may actually be reduced despite the slow metabolism, unlike the increased appetite sometimes seen with other causes of weight loss. Animals may separate from the group and show decreased social interaction. Mental dullness may be apparent, with affected animals responding more slowly to stimuli.

Physical signs of hypothyroidism extend beyond the characteristic goiter to include poor body condition and changes in skin and coat. Despite reduced activity and sometimes reduced feed intake, affected animals typically maintain or gain weight due to their slowed metabolism, though the weight consists largely of water retention and myxedematous tissue rather than productive weight. Skin becomes thickened and may feel doughy or non-pitting when pressed. Hair or wool coats become dull, dry, and sparse. Growth is delayed in young animals. Body temperature may be subnormal. Heart rate is typically low (bradycardia). Reproductive organs may be underdeveloped in young animals or atrophied in adults. Anemia may develop due to reduced oxygen requirements and decreased erythropoietin production.

Symptom progression in untreated hypothyroidism follows a gradual course as thyroid hormone levels continue to decline. Goiter may become increasingly prominent over time as the thyroid gland continues to enlarge in response to TSH stimulation. Coat quality and skin changes become more pronounced. Reproductive failure becomes increasingly apparent, with complete anestrus in females and decreased libido and fertility in males. In pregnant animals, fetal effects become increasingly severe with prolonged deficiency, potentially resulting in fetal death or birth of weak, goitrous offspring. Immunosuppression increases susceptibility to infectious diseases. Severely affected animals may develop myxedema coma in extreme cases, characterized by profound depression, hypothermia, and potentially death.

Emergency symptoms requiring immediate intervention are relatively uncommon in adult animals with hypothyroidism, as the condition typically progresses slowly. However, neonates born with severe congenital hypothyroidism represent emergency situations, as these animals may be too weak to stand, nurse, or thermoregulate and require immediate supportive care. Neonates with large goiters may experience respiratory distress due to tracheal compression. Any pregnant animal showing signs of dystocia with a suspected goitrous fetus requires veterinary intervention. Adult animals showing profound weakness, severe hypothermia, or collapse should receive emergency veterinary assessment and supportive care.

Clinical examination of animals suspected of having hypothyroidism begins with a thorough physical assessment including careful palpation of the thyroid gland region in the neck. Enlargement of the thyroid gland (goiter) is often the most obvious clinical finding and can range from subtle to extremely prominent. The veterinarian assesses overall body condition, evaluates coat quality and skin texture, measures heart rate and body temperature, and looks for characteristic signs such as myxedema. A detailed history is obtained regarding nutritional management, geographic location, mineral supplementation practices, and any recent dietary changes. The timeline of clinical sign development and any effects on reproduction or production are documented. Examination of other animals in the herd helps determine if the problem is affecting multiple individuals, which would suggest a common nutritional or environmental cause.

Diagnostic tests for hypothyroidism involve measurement of thyroid hormone concentrations in blood samples. Total and free thyroxine (T4) levels are the most commonly measured parameters, with species-specific reference ranges used for interpretation. Depressed T4 levels in conjunction with appropriate clinical signs support a diagnosis of hypothyroidism. Triiodothyronine (T3) may also be measured but is subject to greater variability. TSH measurement, when available for the species in question, may show elevation consistent with the pituitary gland attempting to stimulate an underactive thyroid. Serum iodine levels can help differentiate iodine deficiency from other causes of hypothyroidism. A complete blood count may reveal mild non-regenerative anemia. Serum biochemistry often shows hypercholesterolemia due to decreased lipid metabolism.

Differential diagnosis for animals presenting with signs suggestive of hypothyroidism includes other conditions that can cause poor coat quality, reduced growth, and decreased production. Protein-energy malnutrition causes similar nonspecific signs of poor condition and reduced productivity. Parasitism, both internal and external, can cause coat changes and reduced performance. Chronic infectious diseases should be considered. Other trace mineral deficiencies, particularly copper and zinc, can affect coat quality and immune function. In animals presenting primarily with reproductive failure, other reproductive disorders should be investigated. The presence of goiter helps differentiate hypothyroidism from many of these alternatives, though dietary iodine excess can paradoxically cause goiter in some cases.

Herd-level diagnostics are particularly valuable when hypothyroidism is suspected, as iodine deficiency typically affects multiple animals sharing the same nutritional program. Sampling several animals for thyroid hormone and/or iodine levels provides a better picture of herd iodine status than testing individuals. Feed and water analysis for iodine content helps identify the source of deficiency. Forage and grain samples from the operation can be tested to determine if local feeds are iodine-deficient. Assessment of mineral supplementation programs, including analysis of mineral products actually being consumed, identifies gaps in iodine provision. Examination of neonates for goiter at birth provides a sensitive indicator of herd iodine status, as fetal thyroid function is highly sensitive to iodine deficiency.

Emergency and immediate treatment for acute presentations of hypothyroidism, such as severely weak neonates with congenital goiter, focuses on supportive care and addressing immediate life-threatening problems. Weak neonates require warmth, assisted nursing or tube feeding of colostrum, and potentially oxygen support if respiratory distress is present. If goiter is causing tracheal compression and respiratory obstruction, emergency intervention may be required. Iodine supplementation should be initiated, typically by injection of iodized oil or oral administration of potassium iodide solution. These emergency measures address immediate needs while longer-term treatment and prevention strategies are implemented.

Medical management of hypothyroidism in farm animals primarily involves correction of iodine deficiency when this is the underlying cause. Oral supplementation with potassium iodide or sodium iodide can rapidly restore iodine status, with clinical improvement often evident within weeks. Injectable iodized oil provides a depot of iodine for gradual release and may be preferred when ongoing dietary supplementation is impractical or when rapid restoration of iodine status is needed. The response to iodine supplementation is typically good, with goiter regression and normalization of thyroid hormone levels occurring over weeks to months. For non-iodine-deficient hypothyroidism, supplementation with levothyroxine (synthetic T4) may be considered, though this is less commonly used in production animals due to cost and withdrawal time considerations.

Surgical options are rarely indicated for hypothyroidism in farm animals. In cases where massive goiters are causing mechanical complications such as tracheal compression, surgical reduction or removal might be considered, though this is uncommon. The focus is typically on medical management through nutritional correction rather than surgical intervention. If goiter is severe enough to warrant surgical consideration, prognosis and economic factors often favor culling rather than surgery in production animals.

Supportive care for animals with hypothyroidism includes ensuring adequate nutrition with appropriate energy levels to support recovery while avoiding excessive weight gain. Protection from cold is important, as hypothyroid animals have impaired thermoregulation. Pregnant animals with hypothyroidism require close monitoring as parturition approaches, as weak or goitrous offspring may be produced. Any concurrent conditions should be addressed. Body condition should be monitored and nutrition adjusted as metabolism normalizes with treatment. As thyroid function improves, animals may require dietary adjustment to prevent excessive weight loss as metabolic rate increases.

Herd treatment protocols are appropriate when iodine deficiency hypothyroidism is diagnosed in multiple animals, indicating a systemic nutritional problem. Immediate interventions include provision of iodized salt blocks or loose iodized salt, incorporation of iodine into mineral supplements, or dietary inclusion of iodine-containing additives. Individual treatment of clinically affected animals with injectable iodine may be provided while longer-term dietary corrections are implemented. The source of the deficiency should be identified, whether inadequate supplementation, high-goitrogen feedstuffs, or other factors, and corrected. All animals in the affected group should receive improved iodine supplementation.

Treatment decisions for hypothyroidism must weigh the costs of treatment against the value of the animal and the likelihood of successful recovery. For most cases of iodine deficiency hypothyroidism, treatment is straightforward and cost-effective, and affected animals can return to normal production. Animals with severe or prolonged hypothyroidism, particularly those with reproductive failure or offspring with congenital abnormalities, may have lasting effects that limit their productivity. For valuable breeding animals, treatment is generally warranted. For commercial animals, the decision may depend on the severity of the condition, the animal's production stage, and the ease of incorporating treatment into normal management. Any medications used must be used according to label directions or under veterinary supervision with appropriate withdrawal time considerations.

Recovery timeline for animals treated for iodine deficiency hypothyroidism is generally favorable, with improvement beginning within days to weeks of initiating iodine supplementation. Thyroid hormone levels typically begin normalizing within one to two weeks of adequate iodine provision, though complete restoration of normal function may take several weeks to months. Goiter regression occurs more slowly, over weeks to months, as the hyperplastic thyroid tissue involutes. Coat quality improvements become apparent over the natural shedding and regrowth cycle, which varies by species. Return of normal reproductive function may take several estrous cycles after thyroid hormone levels normalize. Overall, the prognosis for recovery from iodine deficiency hypothyroidism is excellent when adequate supplementation is provided.

Post-treatment care and monitoring include verification that thyroid hormone levels have normalized through follow-up blood testing several weeks after initiating treatment. Iodine supplementation should continue on an ongoing basis to prevent recurrence, typically through improved mineral supplementation programs rather than continued therapeutic dosing. Animals should be monitored for resolution of clinical signs including coat improvement, normalization of activity levels, and restoration of reproductive function. Body condition should be tracked, as treated animals may require dietary adjustment as metabolism normalizes. Any animals that do not respond as expected should be re-evaluated for other concurrent conditions or other causes of thyroid dysfunction.

Prognosis for animals with hypothyroidism depends on the severity and duration of the condition before treatment, the underlying cause, and whether irreversible damage has occurred. Animals with mild, short-duration iodine deficiency have excellent prognoses for complete recovery. Those with prolonged, severe deficiency may have persistent effects on growth, reproduction, or production even after thyroid function normalizes. Animals with genetic causes of hypothyroidism may require ongoing management. Neonates born with severe congenital hypothyroidism may survive with supportive care but may have permanent developmental effects. Fetal losses due to maternal hypothyroidism cannot be recovered, making prevention in future pregnancies the focus.

Return to production considerations following recovery from hypothyroidism include expectation of gradual improvement in milk production, growth rate, and reproductive performance as thyroid function normalizes. Full restoration of production may lag behind normalization of thyroid hormone levels by weeks to months as body systems recover from the effects of hormone deficiency. Breeding animals should not be mated until thyroid status has normalized to avoid reproductive failure and reduce risk of affected offspring. Dairy animals may resume expected production over several weeks. Animals recovering from hypothyroidism should be maintained on appropriate iodine supplementation indefinitely to prevent recurrence.

Vaccination protocols are not applicable to hypothyroidism prevention as this is a metabolic disorder rather than an infectious disease. However, maintaining overall herd health through appropriate vaccination programs supports immune function and reduces the metabolic demands associated with fighting infections. Animals with subclinical hypothyroidism may have impaired immune responses, making vaccination-preventable diseases more important to control.

Biosecurity measures are not directly relevant to hypothyroidism prevention, though good general health practices support overall endocrine function. Preventing introduction of contaminated feeds or water that might contain goitrogenic compounds or contaminants affecting thyroid function could be considered a biosecurity-related prevention measure.

Nutritional prevention is the cornerstone of hypothyroidism control, focusing on ensuring adequate dietary iodine intake for all classes of livestock. The iodine requirement for most livestock species ranges from 0.1 to 0.8 mg/kg of diet dry matter, with higher requirements for pregnant and lactating animals. Iodized salt containing 0.007% iodine should be provided free-choice or incorporated into complete feeds. Trace mineralized salt or complete mineral supplements should specify iodine content and be formulated to meet species-specific requirements. Special attention to iodine supplementation is needed in known iodine-deficient regions and for animals consuming goitrogenic feedstuffs. Seaweed-derived supplements can provide iodine but should be used carefully as iodine content is highly variable.

Management practices for preventing hypothyroidism include regular evaluation of mineral supplementation programs to ensure adequate iodine provision. Feed analysis can identify iodine-deficient forages that require supplementation. When feeding brassica crops or other known goitrogenic feeds, additional iodine supplementation may be needed to offset the goitrogenic effects. Rotation of pastures and diversification of feed sources may reduce risk in areas with variable soil iodine content. Pregnant animals, particularly those carrying multiples, should receive particular attention to iodine status due to their higher requirements. Regular monitoring of neonates for goiter at birth provides an early warning of herd iodine deficiency before adult animals show clinical signs.

Quarantine and testing protocols can include evaluation of iodine status in newly acquired animals, particularly those coming from unknown nutritional backgrounds or iodine-deficient regions. Animals entering the herd should be transitioned to the operation's mineral supplementation program to ensure adequate iodine intake. Blood or serum iodine testing or thyroid hormone measurement can assess iodine status in incoming animals if there is concern about deficiency. Pregnant animals acquired close to parturition represent particular risk if their pre-purchase iodine status was inadequate, as fetal effects may already have occurred.

Daily management and monitoring for prevention of hypothyroidism centers on ensuring consistent access to appropriate iodine supplementation. Salt and mineral feeders should be checked daily to ensure they are accessible and contain adequate supply. Consumption of mineral supplements should be monitored to verify that animals are actually ingesting adequate amounts. Observation of animals for early signs of thyroid dysfunction, including changes in coat quality, activity levels, or condition, allows for early intervention if problems develop. Particular attention should be paid to pregnant and lactating animals, which have higher iodine requirements. Any animals showing potential signs of hypothyroidism should be evaluated by a veterinarian.

Housing and environmental management considerations for preventing hypothyroidism include ensuring that mineral feeders are appropriately placed and protected from weather to maintain palatability and prevent iodine loss from mineral supplements. Salt and mineral sources should be available in both housing areas and on pasture. In cold climates, attention to thermoregulation support is important, as subclinical hypothyroidism impairs cold tolerance. Appropriate ventilation and temperature management support overall metabolic health. Environmental sources of goitrogens should be identified and controlled when possible.

Herd health programs addressing thyroid health should include regular review of mineral supplementation programs with attention to iodine provision. Working with a livestock nutritionist to balance rations and verify adequate iodine intake is valuable. Annual or periodic testing of a sample of animals for thyroid hormone levels can identify subclinical problems before clinical disease develops. Examination of neonates for goiter should be standard practice at births, with any goitrous neonates prompting review of the dam's nutrition. Tracking of reproductive performance, growth rates, and production metrics may reveal subtle effects of subclinical hypothyroidism that warrant investigation.

Record keeping and monitoring for thyroid health should document mineral supplement usage and consumption rates, any iodine supplementation provided, and any cases of clinical hypothyroidism or goiter observed in the herd. Reproductive records may reveal patterns consistent with subclinical thyroid problems. Growth rates of young stock should be tracked. Production records for dairy animals or wool production for sheep can indicate metabolic problems including thyroid dysfunction. Feed sources and any changes in feeding programs should be documented. Results of any diagnostic testing for thyroid function or iodine status should be maintained.

Economic considerations for hypothyroidism prevention and management favor investment in adequate mineral supplementation as a highly cost-effective prevention measure. The cost of providing appropriate iodine supplementation is minimal compared to the potential losses from reduced reproduction, decreased production, weak neonates, and treatment costs associated with clinical hypothyroidism. Iodized salt and complete mineral supplements are widely available and add minimal cost to feeding programs. Regions with known iodine deficiency have well-established supplementation protocols that should be followed. The return on investment for adequate trace mineral nutrition, including iodine, is among the highest of any herd health intervention.

High-risk breeds and species for hypothyroidism are largely determined by geographic location and nutritional management rather than inherent breed susceptibility, though some genetic factors exist. All farm animal species are susceptible to iodine deficiency hypothyroidism when adequate iodine is not provided in the diet. Certain goat breeds, including Boer and Spanish goats, have been reported to have hereditary dyshormonogenetic goiter due to genetic defects in thyroid hormone synthesis. Similar hereditary conditions have been documented in some sheep breeds. Highly productive dairy breeds may have higher iodine requirements due to iodine secretion in milk, potentially increasing their risk when supplementation is marginal. Breeds adapted to iodine-deficient regions may have some degree of physiological adaptation, though this should not be relied upon as a substitute for proper nutrition.

Production type considerations influence hypothyroidism risk primarily through effects on iodine requirements and management intensity. Dairy cattle and dairy goats have elevated iodine requirements during lactation due to iodine secretion in milk, with reported concentrations of 50-100 μg/L in cow's milk under adequate supplementation. Pregnant animals of all species have increased requirements due to fetal needs. Growing animals have higher requirements relative to body size. Extensively managed animals on rangeland or pasture may have limited access to mineral supplementation compared to intensively managed counterparts. Animals in intensive confinement systems typically have more consistent mineral supplementation but may be fed stored feeds from iodine-deficient regions.

Genetic selection and testing for hereditary forms of hypothyroidism is possible in breeds known to carry defects in thyroid hormone synthesis pathways. Animals with hereditary dyshormonogenetic goiter should not be used for breeding, and affected bloodlines should be avoided. For the more common nutritional hypothyroidism, genetic testing is not relevant, and management focuses on nutritional adequacy rather than genetic factors. Some research suggests heritable variation in iodine utilization efficiency and thyroid function, but specific genetic markers for selection are not currently used in commercial livestock production. Maintaining genetic diversity within herds helps preserve normal physiological variation in thyroid function.

Commonly co-occurring conditions with hypothyroidism often involve other trace mineral deficiencies, as animals with inadequate iodine supplementation may also be deficient in other essential minerals. Selenium deficiency commonly occurs alongside iodine deficiency in some geographic regions and causes white muscle disease, retained placenta, and immune dysfunction. Copper deficiency may occur concurrently and affects coat color, growth, and immune function. Zinc deficiency causes skin lesions and poor coat quality that may compound the effects of hypothyroidism. Multiple trace mineral deficiencies may occur when overall mineral supplementation is inadequate. The immunosuppressive effects of hypothyroidism may increase susceptibility to infectious diseases.

Conditions with similar symptoms to hypothyroidism include other causes of poor coat quality, reduced growth, and decreased production. Protein-energy malnutrition causes similar nonspecific signs and may occur concurrently with mineral deficiencies. Parasitism, both internal and external, causes coat changes, reduced growth, and decreased production. Chronic infectious diseases including Johne's disease in ruminants should be considered. Copper deficiency causes some similar clinical signs including rough coat and reduced production. Hyperthyroidism, while causing opposite metabolic effects, may present with goiter in some cases. Other reproductive disorders should be considered when reproductive failure is the primary presenting problem.

Complications and sequelae of hypothyroidism include reproductive failure, which may result in significant economic losses through reduced calf, lamb, or kid crops. Neonates born to hypothyroid dams may have permanent developmental abnormalities or may not survive. Growth retardation in young animals may have lasting effects on adult size and productivity. Immune suppression increases susceptibility to various infectious diseases and may reduce vaccine efficacy. Prolonged severe hypothyroidism in adults may cause permanent changes in body composition and condition. In pregnant animals, severe hypothyroidism can cause stillbirths, dystocia from oversized goitrous fetuses, or birth of weak offspring with congenital hypothyroidism that may not survive.