Hypothyroidism

Hypothyroidism is the mirror image of hyperthyroidism: instead of too much thyroid hormone, the body has too little. The thyroid gland, sitting in the neck, produces hormones that set the pace of metabolism throughout the body. When output falls, everything slows down — heart rate, energy, appetite for activity, and the turnover of skin and coat.

In cats this condition is genuinely uncommon, which is worth emphasising because hyperthyroidism (an overactive thyroid) is one of the most frequent diseases of older cats. The great majority of feline hypothyroidism cases are actually a side effect of successfully treating that opposite problem. Radioactive iodine, surgical removal of thyroid tissue, or long-term anti-thyroid drugs can occasionally push hormone levels below normal.

The clinical picture is one of slowing down. Affected cats sleep more, gain weight without eating more, and develop a dull, greasy or thinning coat because hair follicles cycle sluggishly. Some seek out radiators and sunny windowsills because a slow metabolism leaves them feeling cold. A markedly slow heart rate may be detected during a physical exam.

Congenital hypothyroidism is a separate scenario seen in kittens born with poorly functioning thyroid tissue. These kittens grow slowly and develop a distinctive form of dwarfism, with a large head, short limbs and delayed opening of the eyes and ears. Diagnosed early and supplemented with thyroid hormone, many do well. For adult cats, treatment is straightforward and inexpensive: a daily levothyroxine tablet restores normal hormone levels, and most signs reverse over several weeks.