You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Hypothyroidism can be classified into 3 different types based upon the site of the primary lesion.

Primary Hypothyroidism

The lesion is in the thyroid gland itself. 95% of canine cases are due to primary hypothyroidism. There are 2 different causes which each account for 50% of these cases :

• Autoimmune thyroiditis (also called lymphocytic thyroiditis)
• Atrophy of the follicles (idiopathic atrophy) 

Hypothyroidism is rare in cats, and it usually follows thyroidectomy for hyperthyroidism which is very common in old cats.

Secondary Hypothyroidism

The primary lesion is in the anterior pituitary gland which secretes reduced amounts of  thyroid stimulating hormone (TSH) and so thyroxine production is not stimulated within the thyroid gland. This accounts for 5% of canine cases. No cats have been reported to have this form.

Tertiary Hypothyroidism

The primary lesion is in the hypothalamus resulting in reduced secretion of thyroid-releasing hormone (TRH) from the pituitary. In theory this form of the disease may occur, but it has not been reported in dog or cats.

Clinically these different forms present with the same signs, but they can be differentiated by selective tests -TSH-stimulation and TRH-stimulation tests.

Updated October 2013