Thyroid hormones control the rate of energy release
(metabolic rate), and disease results when they
malfunction. Excess storage of surplus energy causes
obesity, one of the most common diseases of Western
The thyroid hormones T3 and T4 are unique in that iodine
(as iodide) is an essential component of both. In most
parts of the world, iodine is a scarce component of soil,
and for that reason there is little in food.
Thyroid hormones interact with intracellular receptors,
and it is this ligand-receptor complex that directly
provides the signal, generally to specific genes whose
rate of transcription is thereby affected.