In-Depth From A.D.A.M. Background

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:

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.

Regulating thyroid function is a complex and important process that involves several 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. (Eighty percent of the body's iodine supply is stored here.) Iodine is the material used to make the hormone thyroxine (T4).

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

Hypothyroidism occurs when thyroxine (T4) levels drop so low that body processes begin to slow down. Hypothyroidism was first diagnosed in the late nineteenth century when doctors observed that surgical removal of the thyroid resulted in the swelling of the hands, face, feet, and tissues around the eyes. They named this syndrome myxedema and correctly concluded that it was the outcome of the absence of substances, thyroid hormones, normally produced by the thyroid gland. Hypothyroidism is usually progressive and irreversible. Treatment, however, is nearly always completely successful and allows a patient to live a fully normal life.

Hypothyroidism is separated into either overt or subclinical disease. That diagnosis is determined on the basis of the TSH laboratory blood tests. The normal range of TSH concentration falls between 0.45 - 4.5 mU/L.

Subclinical, or mild, hypothyroidism (mildly underactive thyroid), also called early-stage hypothyroidism, is a condition in which thyrotropin (TSH) levels have started to increase in response to an early decline in T4 levels in the thyroid. However, blood tests for T4 are still normal. The patient may have mild symptoms (usually slight fatigue) or none at all. Mildly underactive thyroid is very common (affecting about 10 million Americans) and is a topic of considerable debate among professionals because it is not clear how to manage this condition.

More here:
Hypothyroidism - In-Depth Report - NY Times Health