Understand the gland

The thyroid gland is an organ which lies just under the Adam's apple (larynx) in the neck.

It produces two iodine-containing hormones, thyroxine (T4) and tri-iodothyronine (T3), which control the rate at which cells break down and build up chemical substances in the body (ie cell metabolism).

Excess thyroid hormones cause an increased rate of breakdown of chemicals in the body. This results in a condition called hyperthyroidism whose symptoms include rapid pulse rate, hyperactivity, jumpiness, anxiety and loss of weight.

Insufficient production of thyroid hormones causes a slowing down of the breakdown of chemicals in the body. This results in hypothyroidism whose symptoms include physical and mental slowing down, sensitivity to cold and weight gain.

The production of thyroid hormones is controlled by the thyroid-stimulating hormone (TSH) from the pituitary gland. There is a feedback mechanism by which the levels of thyroid hormone in the blood also controls the pituitary, reducing or increasing the output of TSH where necessary.