Which gland in the neck controls metabolism?

The thyroid gland in the neck controls metabolism. This butterfly-shaped endocrine gland produces hormones that help regulate how quickly cells use energy and influence body temperature, heart rate, digestion, and growth.

The thyroid mainly releases thyroxine, known as T4, and triiodothyronine, known as T3. T4 is produced in larger amounts, while T3 is more biologically active. Much of the body’s T4 is converted into T3 in tissues outside the thyroid. Production is controlled through signals from the hypothalamus and pituitary gland, a feedback system called the hypothalamic–pituitary–thyroid axis.

The thyroid requires iodine to manufacture its hormones. Too little iodine can impair hormone production, while autoimmune conditions can cause the gland to become overactive or underactive. Hyperthyroidism may produce weight loss, rapid heartbeat, and heat intolerance; hypothyroidism can cause tiredness, weight gain, and sensitivity to cold.

The parathyroid glands are a common mix-up. They sit behind the thyroid and primarily regulate blood calcium, not overall metabolic rate. The adrenal, pituitary, and thyroid glands all have important roles, but the thyroid is the canonical answer to this question.