Feline Hyperthyroidism / T4 (Cats)

What is hyperthyroidism?

Feline hyperthyroidism is not an infectious disease, but rather an endocrine disorder of the thyroid gland. It is almost always caused by a benign nodular lesion—adenomatous hyperplasia or an adenoma — in one or both lobes of the thyroid gland, which leads to autonomous, excessive production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). In about 70% of cases, both thyroid lobes are affected; malignant thyroid carcinoma is rare in cats. T4 is the hormone predominantly secreted and circulating in the blood; it is converted in peripheral tissues into the biologically more active T3.

Prevalence in Older Cats

Since this is a non-contagious endocrine disorder, transmission of pathogens is not a factor. Epidemiologically, hyperthyroidism is the most common hormonal disorder in older cats; it predominantly affects animals aged seven years or older, with increasing prevalence in older cats. The exact cause of the adenomatous changes has not yet been conclusively determined.

Symptoms of Hyperthyroidism

The clinical signs reflect the increased metabolism. Typical symptoms include weight loss despite increased appetite, restlessness and hyperactivity, increased thirst and urination, as well as vomiting and diarrhea. Cardiovascular changes are frequently observed, such as an elevated heart rate, heart murmurs, and, in advanced cases, signs of cardiac strain. Less commonly, an apathetic form of the disease presents with reduced appetite and lethargy. Successful treatment may reveal a previously undiagnosed kidney disease, which is why kidney function is monitored throughout the course of treatment.

Diagnosis Based on T4 Levels

The diagnosis is based on a combination of typical clinical signs, a palpable thyroid nodule, and an elevated concentration of total T4 in the blood; T4 measurement is therefore the key laboratory parameter. It should be noted that some hyperthyroid cats—especially in early or mild stages—may have T4 levels within the (upper) reference range; therefore, follow-up monitoring or further diagnostic testing is advisable if clinical signs are present but a single test result is normal. Concomitant conditions can influence T4 levels.

Rapid Test Component

The Fassisi® ThyroControl Cat measures the thyroid hormone T4 in serum or plasma and aids in the diagnosis of hyperthyroidism in cats. It is suitable as a point-of-care test in cases of clinical suspicion as well as for monitoring the course of the disease; abnormal findings are evaluated within the overall clinical picture.

References are listed on the References page.