What is the medical term for an underactive thyroid gland?

The medical term for an underactive thyroid gland is hypothyroidism. It occurs when the thyroid does not produce enough thyroid hormones, especially thyroxine (T4) and triiodothyronine (T3), which help regulate metabolism, energy use, temperature, and many organ systems.

The most common cause in many countries is Hashimoto’s thyroiditis, an autoimmune condition in which the immune system gradually damages thyroid tissue. Other causes include iodine deficiency, thyroid surgery, radioactive iodine treatment, certain medicines, and inflammation of the thyroid. Doctors commonly evaluate the condition with blood tests for thyroid-stimulating hormone (TSH) and thyroxine.

Typical symptoms can include tiredness, feeling unusually cold, constipation, dry skin, muscle aches, a slow heart rate, depression, and weight gain. These symptoms can overlap with other illnesses, so they do not establish the diagnosis by themselves. Hypothyroidism is different from hyperthyroidism, in which the thyroid produces too much hormone. Standard treatment is usually levothyroxine, a synthetic form of thyroxine.