The 2 primary thyroid hormones are triiodothyronine (T3) and thyroxine (T4). These hormones are synthesized and secreted by the thyroid, and they are responsible for stimulating metabolism in most cells of the body. Their secretion is regulated primarily by thyroid-stimulating hormone (TSH), which is produced by the pituitary gland. Thyroid-stimulating hormone is regulated by thyroid-releasing hormone (TRH; which is stimulatory) and somatostatin (which is inhibitory). Both of these hormones are produced by the hypothalamus. T3 is far more active than T4, so most T4 is converted to T3 in the periphery (e.g., liver). T3 helps to maintain normal cellular metabolism, oxygen consumption, energy levels, heart rate, thermoregulation, bowel movements, mental health, and neurologic function. Abnormalities in thyroid hormone levels can lead to hyperthyroidism and hypothyroidism.