Which gland in the neck regulates the body's metabolism?

The thyroid gland in the neck regulates the body's metabolism by producing thyroid hormones.

The thyroid is a butterfly-shaped endocrine gland located in front of the trachea, just below the larynx. Its follicular cells make thyroxine, or T4, and triiodothyronine, or T3. These hormones influence how quickly cells use energy and also affect heart rate, body temperature, digestion, muscle function, and growth.

The gland operates as part of the hypothalamic–pituitary–thyroid axis. The hypothalamus releases TRH, prompting the pituitary gland to release thyroid-stimulating hormone, or TSH. TSH signals the thyroid to produce hormones, and rising thyroid-hormone levels feed back to reduce further stimulation. Iodine is essential because the thyroid uses it to build T3 and T4.

An underactive thyroid, called hypothyroidism, can slow metabolism and cause tiredness, cold intolerance, and weight gain. An overactive thyroid, or hyperthyroidism, can produce weight loss, heat intolerance, tremor, and a rapid heartbeat. The pituitary regulates the thyroid, but it is not the gland that directly produces thyroid hormones.