Hypothyroidism is a disorder of the thyroid gland, a small gland in the neck. Its primary function is to regulate the metabolism. With hypothyroidism, the thyroid is under active, so the metabolism is decreased. Hypothyroidism is rare in cats as opposed to hyperthyroidism (note the er) which is far more common. In fact, hypothyroidism is most likely to occur in a cat who is being treated for hyperthyroidism or after thyroid surgery. The treatment works too well and the thyroid goes from being overactive to being under active. Occasionally hypothyroidism may be seen in older cats, but again, it is rare.