About Feline Hyperthyroidism

Hyperthyroidism was not recognized as a clinical problem in cats until 1979. It is now considered the most common endocrinopathy in cats.

Hyperthyroidism is mainly seen in middle-aged to older cats, with a mean age of 12-13 years. There is no sex or breed predilection, however some studies suggest that Siamese and Himalayan cats might have a decreased risk.

Anatomy and Physiology of the Thyroid Gland

The lobes of the thyroid gland are loosely attached, caudolaterally, to the trachea in the cervical region. In cats a normal thyroid gland is usually not palpable.

The thyroid gland synthesizes and secretes the hormones thyroxine (T4) and tri-iodothyronine (T3). Synthesis and secretion are regulated by the secretion of pituitary thyrotropin (thyroid stimulating hormone, TSH).

Thyroid hormone is essential for the normal growth and development of the neurological and skeletal systems.
It also has a wide variety of physiologic effects, including:

Increasing the metabolic rate and oxygen consumption of most tissues
Positive inotropic and chronotropic effects in the heart
Catabolic effects in muscle and adipose tissue
Stimulation of erythropoiesis

Pathophysiology

Hyperthyroidism is a multisystemic disorder caused by the excessive secretion of T4 and T3. This has a negative feedback effect on the pituitary gland, suppressing the release of thyrotropin (thyroid stimulating hormone, TSH) and resulting in atrophy of the normal thyroid tissue.

“Hyperthyroidism is considered the most common endocrinopathy in cats.”