Hypothyroidism

Signs develop gradually and are often vague. Owners first notice lethargy, mental dullness, exercise intolerance and weight gain, then coat changes: dryness, excess shedding, poor regrowth and symmetrical hair loss, part of the alopecia present in 93% of Egyptian cases and the form seen in the severe ones, sometimes with pyoderma and a puffy face from myxoedema. Other findings include infertility, neurological signs such as facial nerve paralysis, laryngeal paralysis, megaoesophagus, ataxia or hind limb weakness, and occasionally cardiac changes. Severe untreated cases can progress to myxoedema coma. Blood tests often show high cholesterol (about 80%) and mild nonregenerative anaemia.

Hypothyroidism is deficient production of thyroid hormone, which lowers cellular metabolic rate throughout the body. About 95% of canine cases are primary, arising in the thyroid gland itself, split roughly equally between lymphocytic thyroiditis, an immune-mediated destruction of the gland often marked by thyroglobulin autoantibodies, and idiopathic follicular atrophy. Secondary disease follows pituitary tumours that destroy the thyrotroph cells producing thyroid-stimulating hormone. Congenital forms result from thyroid dysgenesis or defective hormone synthesis. In cats, the most common form is iatrogenic, following radioiodine, surgery or antithyroid drugs for hyperthyroidism. Slowed metabolism causes weight gain and lethargy, impaired hair growth, raised blood cholesterol, nonregenerative anaemia, and neuromuscular and reproductive dysfunction.

Hypothyroidism is one of the most commonly diagnosed endocrine diseases of dogs but is rare in cats and horses. True prevalence in dogs is estimated at about 1%, and it is overdiagnosed: in a UK primary-care study, specialists judged only 38-56% of dogs receiving thyroid supplementation to be confirmed or likely hypothyroid. It is most common in dogs aged 4-10 years with no sex predilection; Golden Retrievers, Doberman Pinschers, Irish Setters, Miniature Schnauzers, Dachshunds, Cocker Spaniels and Airedales are predisposed. In Egypt, 14% of 200 dogs presented for alopecia or obesity were hypothyroid, mostly middle-aged, with Golden Retrievers, Griffons, German Shepherds and Labradors most represented.

The main mimic is nonthyroidal illness (euthyroid sick syndrome), in which concurrent disease lowers thyroid hormone levels in a dog with a normal gland; drugs such as glucocorticoids and phenobarbital, old age and breeds such as Greyhounds with lower reference ranges also give falsely low total T4. Other causes of alopecia, obesity and lethargy must be excluded. Testing should be reserved for dogs with compatible signs, combining total T4, free T4 by equilibrium dialysis and TSH; scintigraphy or retesting after concurrent illness resolves helps in doubtful cases.

Thyroid gland, pituitary gland, skin and hair coat, cardiovascular system, peripheral nerves and larynx, oesophagus, reproductive organs, blood

With lifelong levothyroxine replacement, treatment failure is uncommon when the diagnosis is correct. Activity improves within weeks; in an Egyptian series, skin and coat recovered fully after 6-8 months. Myxoedema coma is a severe complication.