deiodinase

Deiodinases are the local activation switches for thyroid hormone. The gland mostly ships out the weak form, T4, and these enzymes in the tissues decide whether to switch it on, leave it off, or shut it down — like dimmer switches in each room rather than one master controller.

By removing a single iodine atom from a specific position, a deiodinase converts T4 into active T3, sharply increasing its effect on the cell. Removing iodine from the other position instead yields reverse T3, which is inactive. There are different deiodinase types in different tissues, so the brain, liver, and brown fat can each tune their own thyroid hormone exposure independently of the blood level.

This local control is why circulating hormone numbers do not tell the whole story. During serious illness or starvation, the body shifts conversion away from T3 and toward reverse T3, dialing metabolism down; this so-called euthyroid sick state usually needs no thyroid treatment because the gland itself is healthy. Some deiodinases also depend on selenium, linking trace-mineral nutrition to thyroid function.