Hypothyroidism (impaired thyroid hormone production) results from inflammation (lymphocytic thyroiditis) or shrinking (idiopathic atrophy) of the gland. The underlying cause of these conditions is unclear. Iodine is a critical component for thyroid production and is in fact the only area of the body which utilizes iodine. Deficiency of iodine (goitre) can result in hypothyroidism however iodine is routinely added to commercial diets so deficiencies are now rare.

Clinical signs of hypothyroidism are wide ranging and relate to reduced cellular metabolism. These include:

Lethargy / dullness

Hairloss and dry dull coat. Hairloss is usually in the form of symmetrical thinning without irritation.

Weight gain

Hypothermia (low body temperature)

Bradycardia (slow heart rate). There may also be a relationship between hypothyroidism and the development of dilated cardiomyopathy (stretching of the heart).

Failure to cycle (intact females)

Anemia

Diagnosis is via measurement of thyroid hormone in the blood. Thyroxine (T4) is the most common hormone tested however the levels of T4 can fluctuate significantly during the day, and may be influenced by other illnesses and some drugs. The thyrotropin stimulating hormone (TSH) stimulation test is a more accurate measure of hypothyroidism. This test involves the administration of the pituitary hormone TSH then measuring T4 to assess whether the responding increase in T4 which occurs in normal animals is present.

3,5,3'triiodothyronine (T3) is another thyroid hormone which may be measured however this test appears to offer no benefits over T4 assay.

Treatment requires administration of thyroid hormone preparations generally for the rest of the animals life. Synthetic L-Thyroxine is the most commonly used compound at a rate of 0.02-0.04mg/kg divided daily dose. Response to treatment is assessed by follow up blood testing.