Hypothyroidism - Introduction

Highlights	Diagnosis	Resources
Introduction	Risk Factors	References
Causes	Complications
Symptoms	Treatment

Introduction

The thyroid is a small, butterfly-shaped gland located in the front of the neck that produces hormones, notably thyroxine (T4) and triiodothyronine (T3), which stimulate vital processes in every part of the body. These thyroid hormones have a major impact on the following functions:

Growth.
Use of energy and oxygen.
Heat production.
Fertility.
The use of vitamins, proteins, carbohydrates, fats, electrolytes, and water.
Immune regulation in the intestine.

The effects of these hormones can also alter the actions of other hormones and drugs.

The thyroid gland, a part of the endocrine (hormone) system, plays a major role in regulating the body's metabolism.

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Iodide and Thyroid Hormone Production

Regulating thyroid function is a complex and important process that involves a number of factors, including iodide and four thyroid hormones. Any abnormality in this intricate system of hormone synthesis and production can have far-reaching consequences on health.

Iodide. An understanding of the multi-step thyroid hormone process begins with iodide, a salt that is extracted from the blood and trapped by the thyroid gland. Iodide is converted to iodine in the thyroid gland. (Here, 80% of the body's iodine supply is stored.) Iodine is the material used to make the hormone, thyroxine (T4).

Thyroid Hormones. Four hormones are critical in the regulation of thyroid function:

Thyroxine (T4) and Triiodothyronine (T3). Thyroxine (T4) is the key thyroid hormone. Low levels of T4 produce hypothyroidism and high levels produce hyperthyroidism. Thyroxine converts to triiodothyronine (T3), which is a more biologically active hormone. Only about 20% of triiodothyronine is actually formed in the thyroid gland. The rest is manufactured from circulating thyroxine in tissues outside the thyroid, such as those in the liver and kidney. Once T4 and T3 are in circulation, the typically bind to substances called thyroid hormone transport proteins, after which they become inactive. (In fact, only 0.03% of thyroxine and 0.5% of triiodothyronine are free, that is, unbound and chemically active.)
Thyrotropin. Thyrotropin (also called thyroid-stimulating hormone or TSH) is another very important hormone in the process. It is secreted by the pituitary gland and directly influences the whole process of iodine trapping and thyroid hormone production. When thyroxine levels drop even slightly, the pituitary gland goes into action to pump up secretion of thyrotropin so that it can stimulate thyroxine production. So, when T4 levels fall, TSH levels increase.
Thyrotropin-releasing hormone (TRH), the final critical thyroid hormone, is produced in a region in the brain called the hypothalamus and monitors thyrotropin levels.