thyroid hormone

Thyroid hormone is the body's master thermostat for metabolism, the dial that sets how fast nearly every cell runs. Too little and everything slows down: you feel cold, tired, sluggish, and gain weight. Too much and everything speeds up: you feel hot, jittery, and your heart races. The thyroid gland in the neck produces it under instruction from the brain.

There are two main forms: thyroxine (T4), the abundant but relatively inactive storage form, and triiodothyronine (T3), the potent active form into which T4 is converted in tissues. Both are iodine-containing molecules that enter cells and act on nuclear receptors, changing gene expression to increase basal metabolic rate, oxygen consumption, heat production, and the sensitivity of the heart to adrenaline-type signals. Their production is governed by thyroid-stimulating hormone from the pituitary, which is in turn controlled by the brain.

Pharmacologically, thyroid hormone is given as replacement therapy when the gland underproduces, a condition called hypothyroidism, almost always using synthetic T4 (levothyroxine). At the opposite extreme, overproduction (hyperthyroidism) is treated by reducing hormone output with antithyroid drugs, radioactive iodine, or surgery. A practical caveat: the therapeutic window is narrow and dose is guided by blood tests, because both under- and over-replacement carry real harm, including bone and heart effects from excess.