Feline Hyperthyroidism / T4 (Cats)

Pathogen / Biology

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 — of one or both thyroid lobes, which leads to autonomous, excessive production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). In approximately 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.

Transmission / Epidemiology

Since this is a non-contagious endocrine disorder, transmission of a pathogen 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 lesion has not yet been conclusively determined.

Course & Symptoms

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

Diagnostic Relevance

The diagnosis is based on the 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 the clinical presentation is consistent and the individual 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.