What Does the Thyroid Do?

Medically Reviewed

The thyroid is an endocrine gland located in the lower front of the neck that is shaped like a butterfly. It produces hormones that are secreted into the blood and carried throughout the body.

Those hormones regulate metabolism—the rate at which the body’s cells convert nutrients and oxygen into energy. This in turn affects every cell, tissue, and organ in your body, as well as critical functions such as heart rate, blood pressure, and body temperature.

Thyroxine (T4) is the main hormone secreted by the thyroid, but it also secretes small amounts of triiodothyronine (T3) in the blood.

The pituitary gland contains cells that measure your T4 level to determine if it is within a “normal” range. Generally a T4 level of 0.4 to 4.0 mU/L is considered normal, although it may be higher in older adults.

The pituitary gland secretes thyroid-stimulating hormone (TSH) to regulate how much T4 the thyroid produces. When T4 levels are low, the pituitary gland secretes more TSH to prompt the thyroid to make more T4.

When T4 levels are high, the pituitary gland secretes less TSH so that the thyroid cuts back on the amount of T4 it produces.


Hypothyroidism most often occurs when the thyroid does not produce enough thyroid hormone (an underactive thyroid). This can be caused by an autoimmune disease such as Hashimoto’s disease, inflammation of the thyroid (thyroiditis), medications such as lithium (when taken for many years) and amiodarone, too little or too much iodine, damage to the pituitary gland, or other factors.

The symptoms of hypothyroidism vary widely, but often include extreme fatigue, low energy, mild weight gain (5 to 20 pounds), dry skin, problems with thinking or memory, and depression. People usually develop these symptoms slowly over months or years.


Hyperthyroidism occurs when the thyroid produces too much thyroid hormone (an overactive thyroid). The most common form of hyperthyroidism, Grave’s disease, is a genetic condition in which antibodies in the blood cause the thyroid to grow and secrete too much hormone.

Other possible causes of hyperthyroidism include nodules or lumps in the thyroid and thyroiditis.

Hyperthyroidism symptoms typically include nervousness, irritability, anxiety, increased sweating, increased heart rate, sleep problems, and muscle weakness.