Hypothyroidism

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Hypothyroidism

Hypothyroidism is one of the most common endocrine (hormonal) disorders of dogs. It is very rare in cats. Breeds that are commonly affected include Airedales, Cocker Spaniels, Dachshunds, Doberman Pinschers, Golden Retrievers, Great Danes, Irish Setters, Miniature Schnauzers, and Old English sheepdogs.

Hypothyroidism is a condition in which the thyroid gland does not produce thyroxin (a thyroid hormone abbreviated T4). The thyroid gland is located on both sides of the trachea (windpipe) just below the larynx (voice box). Thyroxin is responsible for regulating many metabolic processes as well as having effects on the heart, respiratory centers, bone, red blood cell formation, and development of young animals. The thyroid gland is activated by stimulation from the pituitary gland and the hypothalamus (areas of the brain that regulate hormonal levels in the body). Hypothyroidism is most commonly due to inflammation of the thyroid gland or atrophy (wasting) of the gland. It can also occur secondary to removal of the gland, due to cancers of the pituitary gland or hypothalamus or it can be congenital (the animal is born with the disease).

An animal that is hypothyroid can have any of the following signs: bilaterally symmetrical hair loss, skin pigmentation, cold intolerance, lethargy, seborrhea (dandruff), constipation, weight gain, and infertility. Rarely, neurological signs may develop. An animal that is hypothyroid may also have other endocrine diseases, such as diabetes and hyperadrenocorticism. Concurrent disease may affect the animal's response to treatment for hypothyroidism. Congenital hypothyroidism often shows up as stunted growth, mental dullness, hair loss, weakness, a short lower jaw, and delayed tooth eruptions.

To diagnose hypothyroidism a blood sample is taken and thyroxin (T4) concentration is measured. A hypothyroid dog would have a low T4 level. Occasionally, the levels of T4 will be in the "gray zone" meaning that the levels are lower than the normal range but are not diagnostic for hypothyroidism. To complicate matters further, if an animal is sick for other reasons, their T4 levels can be low resulting in a condition known as "sick euthyroid syndrome". In either of these cases (a "gray zone" value or "sick euthyroid" animal), a Free T4 by Equilibrium Dialysis (FreeT4ED) can be performed to better define the thyroid status of the animal.

Treatment for hypothyroidism is relatively straightforward. Treatment involves supplementation with L-thyroxin (Soloxine® or Thyro-tabs®). Any secondary skin disorder should be treated as well. Typically, attitude and activity improve within one week after treatment is started, and hair loss and dull hair coat improve within six weeks. A T4 should be repeated 2-4 weeks after initiating therapy or after changing a dosage. A T4 should also be checked if there is inadequate response or if an overdose is suspected (panting, nervousness, aggression, rapid weight loss, or excessive appetite).

The prognosis for an animal with hypothyroidism due to inflammation or an atrophied gland is excellent with proper treatment. Congenital hypothyroidism or hypothyroidism due to a pituitary or hypothalamic mass carry a guarded prognosis.