Thyroid hormone is the collective name for the hormones produced by the thyroid gland — principally thyroxine (T4) and the more active triiodothyronine (T3) — that regulate the body’s metabolic rate. Released into the bloodstream, they act on virtually every tissue, controlling how fast cells consume oxygen and burn fuel, influencing heart rate, body temperature, growth, and brain development. Production is governed by a feedback loop with the pituitary gland, and the hormones are built from the amino acid tyrosine plus iodine, which is why iodine nutrition is critical. Too little causes hypothyroidism’s characteristic slowing; too much causes hyperthyroidism’s racing. Synthetic thyroxine is one of the world’s most prescribed medicines.