Hypothyroidism

What it is

In the dog, hypothyroidism is almost always primary, caused by loss of thyroid tissue. The two main forms are lymphocytic thyroiditis, an immune-mediated destruction of the gland with antibodies against thyroglobulin, and idiopathic follicular atrophy, in which the gland is replaced by fat with little inflammation; the second may be an end stage of the first. Secondary (pituitary) and congenital hypothyroidism are rare. Lack of thyroxine (T4) and triiodothyronine (T3) slows the metabolism of almost every tissue.

Who it affects

Middle-aged dogs of medium to large breeds. Breeds with a reported higher risk include the golden retriever, Dobermann, Irish setter, boxer, beagle, English setter, cocker spaniel and Great Dane.

Anatomy involved

• Thyroid gland (glandula thyroidea): two separate lobes on the lateral surfaces of the trachea, at the level of the first tracheal rings, caudal to the larynx; in the dog the isthmus is usually absent or vestigial. In hypothyroidism the lobes become small and are not palpable.
• Skin: thyroid hormones are needed to start the hair growth cycle; their lack causes alopecia and hyperkeratosis.

Clinical signs

Signs appear gradually and are often attributed to ageing:

• Metabolic: lethargy, mental dullness, exercise intolerance, weight gain without increased appetite, heat-seeking (cold intolerance).
• Dermatological: bilaterally symmetrical, non-pruritic alopecia of the trunk, flanks and tail ("rat tail"), failure of hair to regrow after clipping, dry and dull coat, hyperpigmentation, seborrhoea and recurrent pyoderma or otitis; thickened facial skin with a "tragic" expression from myxoedema.
• Others: bradycardia, peripheral neuropathies, and in rare cases myxoedema coma.

Diagnosis

• Routine blood work: mild non-regenerative anaemia and hypercholesterolaemia are typical.
• Total T4: a normal value makes hypothyroidism unlikely, but a low value is not enough on its own, because illness and many drugs (glucocorticoids, phenobarbital, sulfonamides) lower it (euthyroid sick syndrome). Sighthounds normally have low T4.
• Free T4 by equilibrium dialysis and endogenous TSH: a low free T4 with a high TSH supports the diagnosis.
• Thyroglobulin autoantibodies: indicate lymphocytic thyroiditis.
• Response to treatment: improvement of signs during supplementation confirms the diagnosis.

Treatment

Lifelong oral levothyroxine (synthetic T4). Activity and mental state improve within days to weeks; the coat and body weight take months. The dose is adjusted by measuring total T4 a few hours after the dose. Over-supplementation can cause thyrotoxicosis (polyuria, panting, weight loss, tachycardia). The prognosis is excellent with treatment.