Iodine Isotopes in the Treatment of Thyroid Conditions

Radioisotopes are used to treat many different medical problems of which thyroid conditions are just one example. Here is some background information.

How does the thyroid gland function?

The thyroid is the largest gland in the neck and secretes thyroid hormones which are responsible for the regulation of the body's metabolism.  The thyroid gland produces three main hormones:

1) Thyroxine (T4)



2) Triiodothyronine (T3)



3) Calcitonin

T4 and T3 control growth and and development as well as energy for metabolism. Calcitonin regulates the level of calcium in blood plasma.

The thyroid gland is governed by two other glands, the pituitary gland and the hypothalamus.  The hypothalamus detects that there are low levels of thyroid hormones in the blood and releases thyrotropin releasing hormone (TRH).  This causes the anterior pituitary gland to secrete thyrotrophin also known as thyroid-stimulating hormone (TSH). TSH along with protirelin, a synthetic tripeptide, stimulates the thryroid gland to produce more hormones and so the level returns to normal.

There is also a mechanism to reduce the thryroid hormone level when it has become too high.  The hypothalamus releases somatostatin which acts on the anterior pituitary gland and inhibits TSH secretion.  T3 and T4 produced from the thyroid gland also cause the anterior pituary to produce less TSH.  However, these negative feedback processes can not inhibit the production of TSH completely.

Overview of thyroid mechansim












inhibitors (agonists)

activators (antagonists)


Rang, Dale, Ritter, Pharmacology, 4th Ed., Churchill Livingstone,1999