Dietary iodine is easily and almost completely absorbed but there is no body store of the element, so that a regular frequent intake is necessary. Excess iodine is excreted in the urine.

The thyroid hormones increase the rate of metabolism of all cells, sometimes by as much as 30 per cent. The hormonal activity is therefore closely related to the metabolic rate of the whole body, which can be measured by the rate at which oxygen is taken up. In the unborn baby, thyroid hormones are essential for proper physical and mental development.