I got mommy to write this article because one of my fursibs is hyperthyroid and I thought it was a good opportunity to write about the hyperthyroidism and its treatment.
SURFEIT IS DIAGNOSED WITH HYPERTHYROIDISM
Surfeit is the senior cat we adopted recently. We were told he was eleven years old. At his first vet visit he checked out fine but the labs showed a borderline T4. The vet ordered a free T4 and that was high, demonstrating that Surfeit is hyperthyroid. We were surprised because he wasn’t what you think of as the ravenous hyperactive cat. He’s actually not a very good eater and we’ve had to give him Periactin (cyproheptadine) to improve his appetite. He was started on Tapazole (methimazole) but blood tests done a month later showed that the medication was affecting his liver and it had to be stopped. This means that radioactive Iodine, commonly called I-131 (131I in proper scientific notation) is the only practical option for him.
THE THYROID AND ITS HORMONES
The thyroid gland is located in the neck. In humans it lies just over the bump below the Adam’s Apple. When your doctor puts a hand on your neck and tells you to swallow, he or she is checking whether your thyroid is enlarged. The thyroid produces and secretes thyroid hormone, which is actually a mixture of several hormones. The major thyroid hormones are T3 and T4, containing three and four iodine atoms, respectively. Thyroid hormones travel in the blood stream bound to special proteins. They are taken up by cells and help regulate many different cell functions including the body’s use of protein, fat and carbohydrate. The usual blood test for T4 measures total T4, including T4 that is bound to its transport protein and the unbound (free) T4. The special free T4 test is more accurate because only the unbound T4 is active.
The thyroid is regulated by the hypothalamus and the pituitary gland via TRH (thyrotropin-releasing hormone) and TSH (thyroid-stimulating hormone or thyrotropin). When the hypothalamus senses that there is not enough thyroid hormone in the body, it secretes TRH, which stimulates the pituitary to secrete TSH. TSH causes the thyroid gland to produce thyroid hormone. When there is enough thyroid hormone in the body, the hypothalamus stops secreting TRH and the pituitary, in its turn, stops TSH secretion. This system of control is called the hypothalamic – pituitary – thyroid axis.
HYPERTHYROIDISM IN CATS
The typical hyperthyroid cat is underweight in spite of a good appetite. It’s hyperactive and may have excessive urination, vomiting and diarrhea as well as an unkempt appearance. The thyroid may be palpably enlarged. There may be a heart murmur, increased heart rate, abnormal heart rhythm and heart failure. The heart failure is generally reversible once the hyperthyroidism is treated but the cat might require medication in the meanwhile. Blood flow to the kidneys is increased, and this may mask pre-existing kidney disease, which becomes apparent once the hyperthyroidism is controlled. Blood pressure may be increased and this may damage many organs, including the kidneys, eyes, brain and heart. A few cats have what is called “apathetic hyperthyroidism,” with decreased activity and appetite. This is the category Surfeit falls into. He’s actually a poor eater and has required appetite-stimulating medication.
Hyperthyroidism most commonly occurs in middle-aged cats and is typically caused by a benign tumor (adenoma) of the thyroid gland. A minority or hyperthyroidism in cats is caused by cancerous (malignant) thyroid tumors. Some cases must be caused by problems with the pituitary or hypothalamus but I haven’t come across a mention of that so it must be very rare indeed.
To be continued.…
Sources and Further reading
Wikipedia links above
Hyperthyroidism in Cats by Cornell Feline Health Center
The Merck Veterinary Manual, Merck & Co., Inc., 2005
Plumb, Donald, Plumb’s Veterinary Drug Handbook, Sixth Edition, Blackwell Publishing, 2008
Feline hyperthyroidism by Advanced Veterinary Medical Imaging
What Is Feline Hyperthyroidism? by Cat Thyroid Center
Erick Mears, DVM, personal communication