Hypothyroidism in Dogs
Hypothyroidism is caused by a decrease in production of thyroid hormone by the thyroid gland. This usually occurs due to either atrophy of the thyroid gland where thyroid tissue is replaced by inactive tissue, or immune mediated inflammation where the immune system inappropriately destroys thyroid tissue. It is very uncommon for hypothyroidism to be due to a tumor or the thyroid gland or surrounding tissue.
This disease typically occurs in older, large breed dogs. Clinical signs are usually vague, affecting multiple organ systems and slow to develop. The changes that appear are often initially attributed to “old age” by owners. Affected dogs tend to be less active, less interested in their surroundings, intolerant of cold and prone to weight gain.
Changes in the skin and hair coat are often noted including dry flakey skin, thinning of hair, dry coat, loss of coat luster and an increase in skin infections. In rare cases some dogs will develop edema in the face causing puffy eyelids, forehead and cheeks.
Reproductive problems and decreased fertility can occur in both males and females if dogs are intact.
Hypothyroidism can also affect the nervous system. Neuropathy can occur leading to weakness, exercise intolerance, ataxia or paralysis. Changes to the vestibular nerve which controls the vestibular system in the ear can cause “stroke like” episodes where the dogs are having trouble righting themselves and are unable to stand. They may have drooping of one side of the face and drooling if the facial nerve is affected.
Congenital hypothyroidism, also called cretinism,is very rare in dogs and leads to failure of these pets to grow properly.
Diagnosis of hypothyroidism is made based on clinical signs and changes on blood work. The most commonly evaluated value is the total T4 which is a good screening test for hypothyroidism. In some cases where the total T4 level is low normal and hypothyroidism is still suspected it may be necessary to evaluate additional thyroid hormone levels.
Treatment is supplementation with a synthetic form of thyroid hormone, usually given twice daily in pill form. Side effects are rare but can include excitability, nervousness and panting if the dose is too high. Activity levels often increase after 1-2 weeks of therapy. Improvements in weight, skin condition and other clinical signs take longer to appear – often 4-8 weeks or longer. 4-8 weeks after starting dogs on supplement, a recheck of thyroid hormone levels is needed to ensure they are at the proper dose. Form there levels are typically checked every 6-12 months or as clinically indicated.