Hypothyroidism

Hypothyroidism is a common endocrine disorder in dogs, characterized by inadequate circulating levels of biologically active thyroid hormones (thyroxine, T4, and triiodothyronine, T3) or impaired tissue response to these hormones. This deficiency leads to a generalized slowing of cellular metabolism, affecting virtually every organ system. In dogs, primary hypothyroidism (thyroid gland failure) accounts for over 95% of cases, while secondary (pituitary) and tertiary (hypothalamic) forms are rare. Feline hypothyroidism is uncommon and typically iatrogenic, following treatment for hyperthyroidism (surgical thyroidectomy, radioactive iodine therapy, or chronic antithyroid medication). Congenital hypothyroidism, though rare, can occur in both dogs and cats due to thyroid dysgenesis or dyshormonogenesis. The clinical syndrome is insidious, with signs developing over months to years, and is often mistaken for other chronic diseases. Early recognition and appropriate hormone replacement therapy result in excellent outcomes, with most clinical signs resolving within weeks to months.

The etiology of hypothyroidism is categorized into primary, secondary, and tertiary forms. Primary hypothyroidism, the most common, results from destruction of the thyroid gland. The two predominant causes are lymphocytic thyroiditis (immune-mediated destruction) and idiopathic thyroid atrophy. Lymphocytic thyroiditis is an autoimmune condition characterized by progressive infiltration of the thyroid gland by lymphocytes, plasma cells, and macrophages, leading to follicular destruction and fibrosis. It is believed to be T-cell mediated, with autoantibodies against thyroglobulin, T4, and T3 often present. Idiopathic thyroid atrophy involves non-inflammatory degeneration of the thyroid follicles, replaced by adipose and fibrous tissue. Less common primary causes include neoplastic infiltration (thyroid adenocarcinoma), infectious agents (bacterial, fungal), and iatrogenic causes such as surgical removal or radioactive iodine therapy. Secondary hypothyroidism results from pituitary gland dysfunction, leading to decreased thyroid-stimulating hormone (TSH) secretion. This can be due to pituitary neoplasms (e.g., macroadenomas), congenital pituitary dwarfism, or trauma. Tertiary hypothyroidism, due to hypothalamic thyrotropin-releasing hormone (TRH) deficiency, is extremely rare. In cats, iatrogenic hypothyroidism is the most common cause, occurring after treatment for hyperthyroidism. Congenital hypothyroidism can arise from thyroid dysgenesis (aplasia, hypoplasia, ectopic tissue) or dyshormonogenesis (enzyme defects in thyroid hormone synthesis).

Hypothyroidism is predominantly a disease of middle-aged to older dogs, with a peak incidence between 4 and 10 years of age. There is no strong sex predilection, though some studies suggest a slight female predisposition. Certain breeds are overrepresented, including Golden Retrievers, Labrador Retrievers, Doberman Pinschers, Dachshunds, Cocker Spaniels, Airedale Terriers, Irish Setters, and Giant Schnauzers. This breed predisposition indicates a genetic component, with heritability estimates for autoimmune thyroiditis ranging from 20% to 40%. The prevalence in dogs is estimated at 0.2% to 0.8% of the general population, but it is higher in certain breeds. In cats, hypothyroidism is rare, with an estimated prevalence of less than 0.5% in the general feline population, but it is more common in cats that have undergone treatment for hyperthyroidism, with up to 30% developing iatrogenic hypothyroidism after radioactive iodine therapy. There is no geographic or seasonal variation reported. Congenital hypothyroidism is rare but can occur in breeds such as Toy Fox Terriers, Scottish Deerhounds, and Giant Schnauzers, often with an autosomal recessive inheritance pattern.

Thyroid hormones (T4 and T3) are essential for normal cellular metabolism, growth, and development. They exert their effects by binding to nuclear thyroid hormone receptors, modulating gene transcription, and influencing mitochondrial function. T4 is the primary secretory product of the thyroid gland, but T3 is the more biologically active form, largely produced by peripheral deiodination of T4. In hypothyroidism, reduced thyroid hormone levels lead to a decrease in basal metabolic rate, impaired thermogenesis, and altered protein, carbohydrate, and lipid metabolism. This results in a characteristic slowing of physiological processes. The clinical signs are largely due to decreased metabolic activity: weight gain without increased appetite, lethargy, exercise intolerance, and cold intolerance. Dermatological manifestations arise from prolonged hair follicle telogen phase, leading to alopecia, poor hair coat, and delayed hair regrowth. Myxedema, a non-pitting edema, results from accumulation of glycosaminoglycans (hyaluronic acid) in the dermis and subcutaneous tissues, drawing in water. Neuromuscular signs, such as peripheral neuropathy and myopathy, are due to altered nerve conduction and muscle metabolism. Reproductive abnormalities, including infertility and prolonged anestrus, are linked to disrupted gonadotropin secretion. Cardiovascular effects include bradycardia and decreased cardiac contractility, due to reduced beta-adrenergic receptor sensitivity. Hypercholesterolemia and hypertriglyceridemia occur because of decreased lipoprotein lipase activity and reduced hepatic clearance of lipoproteins. In congenital hypothyroidism, thyroid hormone deficiency during development leads to impaired skeletal maturation, delayed growth, and central nervous system abnormalities, resulting in dwarfism and intellectual disability.

Predisposing factors for hypothyroidism include genetic susceptibility, particularly in breeds with a high incidence of autoimmune thyroiditis. The presence of circulating thyroglobulin autoantibodies (TgAA) is a risk factor for future development of clinical hypothyroidism, as dogs with positive TgAA have a higher likelihood of progressing to overt disease. Age is a significant factor, with middle-aged to older dogs being more commonly affected. Concurrent autoimmune diseases, such as hypoadrenocorticism (Addison's disease), diabetes mellitus, and immune-mediated polyarthritis, may increase the risk of developing hypothyroidism due to shared genetic and immunological mechanisms. Environmental factors, such as exposure to certain drugs (e.g., sulfonamides, glucocorticoids, phenobarbital) can suppress thyroid hormone levels, though this is usually transient and reversible. Iatrogenic causes, including surgical thyroidectomy, radioactive iodine therapy, and high-dose antithyroid medications, are direct predisposing factors in cats. Nutritional deficiencies, such as iodine deficiency, are rare in commercial diets but can occur with homemade diets. Stress and systemic illness can also transiently lower thyroid hormone levels, but these are not true predisposing factors for permanent hypothyroidism.

The clinical signs of hypothyroidism are insidious and often nonspecific, developing over months to years. They can be categorized into systemic, dermatological, neurological, cardiovascular, reproductive, and gastrointestinal manifestations. Systemic signs include lethargy, exercise intolerance, weight gain without increased appetite, cold intolerance, and mental dullness. Dermatological signs are the most common presenting complaints and include bilateral symmetrical non-pruritic alopecia, particularly over the trunk, tail (rat tail), and flanks. The hair coat is dry, brittle, and may have a dull appearance. Hyperpigmentation, seborrhea, and pyoderma are common secondary findings. Myxedema, a non-pitting edema of the face and skin, is a classic but less common feature. Neurological signs include peripheral vestibular disease, facial nerve paralysis, laryngeal paralysis, and peripheral neuropathy, manifesting as weakness, ataxia, and proprioceptive deficits. Myopathy can cause muscle weakness and exercise intolerance. Cardiovascular signs include bradycardia, weak apex beat, and decreased cardiac contractility, which may be detected on echocardiography. Reproductive abnormalities include infertility, prolonged anestrus, and decreased libido. Gastrointestinal signs, such as constipation and diarrhea, are less common. In congenital hypothyroidism, signs include disproportionate dwarfism, delayed dental eruption, constipation, and mental dullness. In cats, iatrogenic hypothyroidism may present with weight gain, lethargy, and dermatological changes, but signs are often mild and may be masked by concurrent hyperthyroidism treatment.

The differential diagnoses for hypothyroidism are broad due to the nonspecific nature of clinical signs. Key differentials include: 1) Hyperadrenocorticism (Cushing's syndrome) – presents with alopecia, pot-bellied appearance, polyuria/polydipsia, and elevated cortisol levels; differentiate with ACTH stimulation test or low-dose dexamethasone suppression test. 2) Alopecia due to sex hormone imbalances (e.g., Sertoli cell tumor, estrogen-responsive dermatosis) – typically affects intact animals; diagnosis via hormone assays and gonadal palpation/ultrasound. 3) Demodicosis – causes alopecia and skin lesions; diagnosed by deep skin scrapings. 4) Dermatophytosis – fungal infection causing circular alopecia and scaling; diagnosed by fungal culture or PCR. 5) Bacterial pyoderma – secondary infection causing pruritus and pustules; diagnosed by cytology and culture. 6) Chronic renal failure – can cause lethargy, weight loss, and poor hair coat; differentiate with renal function tests (creatinine, SDMA, urinalysis). 7) Hepatic disease – may cause weight loss, lethargy, and skin changes; differentiate with liver enzymes and bile acids. 8) Cardiac disease – can cause exercise intolerance and lethargy; differentiate with echocardiography. 9) Neurological disorders (e.g., myasthenia gravis, polyneuropathy) – may present with weakness and exercise intolerance; differentiate with specific neurological testing. 10) Chronic infections or neoplasia – can cause nonspecific signs; rule out with appropriate diagnostics. Definitive diagnosis of hypothyroidism requires thyroid function testing, including baseline T4, TSH, and possibly free T4 by equilibrium dialysis.

The diagnostic algorithm for hypothyroidism begins with a thorough history and physical examination, focusing on characteristic clinical signs. If hypothyroidism is suspected, baseline serum thyroid hormone testing is performed. A low total T4 (TT4) concentration (<1.0 μg/dL in dogs) is suggestive but not diagnostic, as non-thyroidal illness (NTI) can lower T4. Therefore, a concurrent TSH measurement is recommended. In dogs, a low TT4 with a high TSH (>0.6 ng/mL) is highly supportive of primary hypothyroidism. If TT4 is low but TSH is normal, further testing with free T4 by equilibrium dialysis (fT4ed) is indicated. A low fT4ed (<0.6 ng/dL) with a high TSH confirms hypothyroidism. If results are equivocal, a TSH stimulation test (or TRH stimulation test) can be performed, though it is less commonly used. In cats, diagnosis is more challenging; a low TT4 with high TSH is suggestive, but fT4ed may be helpful. Additional tests include thyroglobulin autoantibodies (TgAA) to support autoimmune thyroiditis. It is important to rule out non-thyroidal illness, as many systemic diseases can suppress thyroid hormone levels. Therefore, a complete blood count, serum biochemistry profile, and urinalysis are recommended to assess overall health and identify concurrent diseases. Imaging (e.g., thyroid scintigraphy) is rarely needed but can be used to assess thyroid gland function. The diagnostic algorithm should be stepwise, starting with baseline tests and progressing to more specific tests if needed.

Laboratory findings in hypothyroidism often include a mild non-regenerative anemia (normocytic, normochromic) due to decreased erythropoietin production and reduced metabolic demand. Serum biochemistry may reveal hypercholesterolemia and hypertriglyceridemia, which are common but not specific. Mild increases in liver enzymes (ALT, AST) may occur due to hepatic lipidosis. Creatine kinase (CK) may be elevated if myopathy is present. Urinalysis is typically unremarkable, but a decreased urine specific gravity may be seen if concurrent renal disease exists. Blood gas analysis is usually normal. Specific biomarkers include low serum total T4 (TT4) and free T4 by equilibrium dialysis (fT4ed), with elevated TSH. In dogs, a TT4 <1.0 μg/dL and TSH >0.6 ng/mL are consistent with hypothyroidism. However, TT4 can be low in non-thyroidal illness, so fT4ed is more specific. Thyroglobulin autoantibodies (TgAA) may be positive in autoimmune thyroiditis. In cats, TT4 <0.8 μg/dL and TSH >0.3 ng/mL are suggestive, but fT4ed is often needed. Additional tests such as serum cholesterol and triglycerides are often elevated. It is important to note that thyroid hormone levels can be affected by drugs (e.g., glucocorticoids, phenobarbital, sulfonamides), so these should be considered when interpreting results.

Imaging is not routinely required for the diagnosis of hypothyroidism, but it may be used to evaluate the thyroid gland or to rule out other conditions. Thyroid scintigraphy using technetium-99m pertechnetate can assess thyroid gland function and morphology. In hypothyroidism, the thyroid gland may appear small or have decreased uptake. However, this is rarely performed in clinical practice. Ultrasonography of the thyroid gland may show reduced size and altered echogenicity, but findings are nonspecific. Thoracic radiographs may be indicated if cardiac disease is suspected, as hypothyroidism can cause bradycardia and decreased cardiac contractility, but these are not specific. Abdominal ultrasound may be performed to rule out other endocrine diseases such as hyperadrenocorticism or to evaluate for concurrent conditions. In cases of congenital hypothyroidism, skeletal radiographs may reveal delayed epiphyseal ossification and shortened long bones. Advanced imaging such as CT or MRI is rarely needed but may be used to evaluate the pituitary gland in suspected secondary hypothyroidism. Overall, imaging plays a supportive role in the diagnostic workup, primarily to exclude other diseases.

Cytology and histopathology are not commonly used for the diagnosis of hypothyroidism, as the diagnosis is primarily based on thyroid function tests. However, if a thyroid mass is present, fine-needle aspiration (FNA) may be performed to differentiate between benign and malignant lesions. Cytology of a thyroid adenocarcinoma may show clusters of pleomorphic epithelial cells with anisocytosis and anisokaryosis. Histopathology of the thyroid gland, obtained via biopsy, can confirm the underlying cause. In lymphocytic thyroiditis, histopathology reveals diffuse infiltration of the thyroid gland by lymphocytes, plasma cells, and macrophages, with destruction of thyroid follicles and fibrosis. In idiopathic thyroid atrophy, the thyroid follicles are small and atrophic, with replacement by adipose and fibrous tissue. Special stains, such as immunohistochemistry for thyroglobulin, can help identify thyroid tissue. However, thyroid biopsy is rarely performed in clinical practice due to the availability of less invasive diagnostic tests. In cases of congenital hypothyroidism, histopathology may show thyroid dysgenesis or dyshormonogenesis.

The treatment of hypothyroidism involves lifelong hormone replacement therapy with synthetic levothyroxine (L-thyroxine). The initial recommended dose for dogs is 0.02 mg/kg (20 μg/kg) administered orally every 12 hours. However, dosing may vary based on individual response and concurrent diseases. For cats, the initial dose is typically 0.05-0.1 mg per cat every 12 hours, but it is often lower in iatrogenic cases. Levothyroxine is available in tablet form, and it is important to administer it consistently with food, as absorption can be affected by diet. The goal of therapy is to normalize clinical signs and maintain serum T4 levels within the reference range (typically 1.5-4.5 μg/dL in dogs) at 4-6 hours post-pill. Monitoring is essential, with serum T4 measured 4-6 hours after administration to assess peak levels. Adjustments are made based on clinical response and T4 levels. If clinical signs do not improve, the dose may be increased by 25% and rechecked in 2-4 weeks. Conversely, if signs of hyperthyroidism (e.g., polyphagia, weight loss, hyperactivity) occur, the dose should be reduced. In dogs with concurrent diseases, such as renal or hepatic insufficiency, lower initial doses may be required. In cats, iatrogenic hypothyroidism may be transient, and some cats may recover thyroid function over time; therefore, periodic reassessment is recommended. Supportive care includes management of secondary skin infections and dietary modifications to address obesity. In congenital hypothyroidism, early treatment is critical to improve growth and development.

The prognosis for hypothyroidism is excellent with appropriate treatment. Most clinical signs, such as lethargy, weight gain, and dermatological changes, improve within 4-8 weeks of initiating levothyroxine therapy. Hair regrowth may take several months, and some dermatological changes may be permanent. Neurological signs, such as peripheral neuropathy, may improve slowly or be partially irreversible. In cases of congenital hypothyroidism, the prognosis is guarded, as early treatment is essential for normal development; delayed treatment may result in permanent growth and neurological deficits. Iatrogenic hypothyroidism in cats generally has a good prognosis, with many cats requiring lifelong therapy, but some may recover thyroid function. The overall mortality rate is low, and with proper monitoring, most dogs and cats can lead a normal quality of life. Negative prognostic indicators include severe neurological deficits, concurrent diseases, and poor owner compliance with medication administration. Regular monitoring and dose adjustments are crucial to prevent complications from over- or under-dosing.

Follow-up for hypothyroidism involves regular monitoring of clinical signs and serum thyroid hormone levels. After initiating levothyroxine therapy, a serum T4 level should be measured 4-6 hours post-pill after 2-4 weeks of treatment. The goal is to achieve a T4 level within the reference range (typically 1.5-4.5 μg/dL in dogs). If the T4 is low, the dose may be increased by 25% and rechecked in 2-4 weeks. If the T4 is high or clinical signs of hyperthyroidism are present, the dose should be reduced. Once a stable dose is achieved, recheck T4 levels every 6-12 months or as needed. Clinical signs should be assessed at each visit, and owners should be educated on the importance of consistent administration. In cats with iatrogenic hypothyroidism, thyroid function should be reassessed every 3-6 months, as some cats may recover. Additionally, concurrent diseases should be monitored, and routine blood work (CBC, biochemistry, urinalysis) is recommended annually. Weight management and skin care are important aspects of long-term follow-up.

Pearls: 1) Always interpret thyroid hormone levels in the context of clinical signs and rule out non-thyroidal illness, as sick euthyroid syndrome can cause low T4. 2) Free T4 by equilibrium dialysis is the most reliable single test for hypothyroidism, as it is less affected by non-thyroidal illness. 3) A high TSH with low T4 is highly specific for primary hypothyroidism. 4) Levothyroxine should be given consistently with food, and peak T4 levels should be measured 4-6 hours post-pill. 5) In cats, iatrogenic hypothyroidism may be transient, so periodic reassessment is warranted. Pitfalls: 1) Do not diagnose hypothyroidism based solely on low T4, as many systemic diseases can lower T4. 2) Avoid using total T4 alone for monitoring; use clinical response and T4 levels. 3) Do not use thyroid hormone supplementation in euthyroid dogs for dermatological conditions, as it can cause iatrogenic hyperthyroidism. 4) Be cautious with dosing in dogs with concurrent diseases, as they may require lower doses. 5) Do not overlook the possibility of concurrent hypoadrenocorticism, as levothyroxine therapy can precipitate an adrenal crisis in such cases.

The primary drug for hypothyroidism is levothyroxine sodium (L-thyroxine). In dogs, the initial dose is 0.02 mg/kg (20 μg/kg) orally every 12 hours. However, the dose may be adjusted based on clinical response and serum T4 levels. For example, a 20 kg dog would receive 0.4 mg every 12 hours. In cats, the initial dose is 0.05-0.1 mg per cat every 12 hours, but it is often lower in iatrogenic cases. Levothyroxine is available in tablet strengths of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.8 mg. It is important to administer the medication consistently with food, as absorption can be affected by diet. Monitoring of serum T4 is recommended 4-6 hours post-pill, with a target range of 1.5-4.5 μg/dL in dogs. If the T4 is below the target, the dose may be increased by 25% and rechecked in 2-4 weeks. If the T4 is above the target or clinical signs of hyperthyroidism (e.g., polyphagia, weight loss, hyperactivity) are present, the dose should be reduced. In dogs with concurrent diseases, such as renal or hepatic insufficiency, a lower initial dose (e.g., 0.01 mg/kg every 12 hours) may be considered. In cats, iatrogenic hypothyroidism may be transient, and some cats may require lower doses or may eventually discontinue therapy. There are no significant drug interactions with levothyroxine, but it may increase the metabolism of other drugs, so monitoring is advised. Contraindications include untreated hypoadrenocorticism, as levothyroxine can increase the metabolic rate and precipitate an adrenal crisis. In such cases, glucocorticoid therapy should be initiated before levothyroxine.

Evidence-based literature supports the use of levothyroxine for the treatment of hypothyroidism in dogs and cats. A landmark study by Scott-Moncrieff et al. (2002) evaluated the diagnostic accuracy of thyroid function tests and found that free T4 by equilibrium dialysis had the highest sensitivity and specificity for diagnosing hypothyroidism. Another study by Kantrowitz et al. (2001) demonstrated that a TSH concentration >0.6 ng/mL in conjunction with a low T4 was highly predictive of hypothyroidism. The ACVIM consensus statement on hypothyroidism in dogs (2018) recommends baseline T4 and TSH measurement, with fT4ed as a confirmatory test. Regarding treatment, a study by Dixon et al. (1999) showed that twice-daily dosing of levothyroxine was more effective than once-daily dosing in maintaining stable T4 levels. A more recent study by Behrend et al. (2006) found that once-daily dosing may be sufficient in some dogs, but twice-daily is generally recommended. In cats, a study by Norsworthy et al. (2013) reported that iatrogenic hypothyroidism after radioactive iodine therapy is common, and treatment with levothyroxine is effective. The prognosis is excellent with appropriate therapy, as demonstrated by a study by Panciera (1994) that showed resolution of clinical signs in most dogs within 2-3 months. Overall, the evidence supports the use of levothyroxine as the standard of care, with careful monitoring to avoid over- or under-dosing.