When the thyroid gland overproduces hormones, it’s called hyperthyroidism, which in the U.S. is most commonly caused by Graves’ disease, an autoimmune disorder. An overactive thyroid revs up your metabolism, causing symptoms such as nervousness, rapid or irregular heart rate, weight loss, irritability, anxiety, insomnia, muscle weakness, tremors, sleep and vision problems, goiter, palpitations, menstrual irregularity, heat intolerance, and impaired fertility. Bulging eyes (exophthalmos) are a sign of Graves’ disease. (In contrast, an underactive thyroid is called hypothyroidism, which is much more common that hyperthyroidism.)
In response to the overproduction of thyroid hormones, the pituitary gland normally lowers levels of thyroid stimulating hormone (TSH), signaling the thyroid to put on the brakes. Thus, a low blood level of TSH usually indicates hyperthyroidism.
Untreated hyperthyroidism can cause a host of cardiovascular disorders—including abnormal heart rhythms (atrial fibrillation), hypertension, angina, and heart failure—as well as reduced bone mineral density. Some recent research has linked even modestly high levels of thyroid hormone (T4) to cardiovascular disease.
Hyperthyroidism can be treated with drugs that block hormone production, but more commonly, the thyroid gland is inactivated with radioactive iodine; after that, oral thyroid hormones must usually be taken.