thyroxine (T4)

Thyroxine is the thyroid’s main product, but it is more of a stored cheque than spendable cash. It is the dominant hormone the gland releases, yet much of its action depends on being converted into a more potent form once it reaches the tissues.

Each thyroxine molecule carries four iodine atoms, which is why it is abbreviated T4. The thyroid releases far more T4 than T3, and T4 circulates for about a week, mostly bound to carrier proteins, giving a steady background supply. In peripheral tissues, deiodinase enzymes remove one iodine to turn T4 into triiodothyronine, the form that binds thyroid hormone receptors strongly; alternatively they can convert it to inactive reverse T3.

Because of this conversion step, T4 is often described as a prohormone with weak direct activity. Its long half-life is exactly why the standard treatment for an underactive thyroid is once-daily levothyroxine, a synthetic copy of T4 that the body steadily converts to T3 on its own schedule.