triiodothyronine (T3)

If thyroxine is the stored cheque, triiodothyronine is the cash that actually does the spending. It is the form of thyroid hormone that binds the receptor tightly and produces most of the real effects on metabolism and growth.

Triiodothyronine carries three iodine atoms, hence T3. Only a minority of circulating T3 comes directly from the gland; most is made inside target tissues when deiodinase enzymes strip one iodine from T4. Once inside the cell, T3 enters the nucleus and acts on a nuclear receptor that turns genes on or off, slowly reshaping how the cell handles energy. Its effects build over hours to days rather than seconds.

Because it acts at the genetic level, T3 influences almost every organ: it speeds the heart, raises heat production, quickens the gut, and is indispensable for normal brain maturation early in life. Measuring T3 in blood is less reliable than measuring T4 and TSH for diagnosing thyroid disease, partly because so much of the body’s T3 is produced and used locally inside cells.