Thyroid Diseases

Thyroid Regulation

The pituitary gland is the master regulator of thyroid function, producing thyroid-stimulating hormone (TSH), which stimulates the thyroid to produce T4 and T3. The hypothalamus, in turn, controls pituitary function by secreting thyroid-releasing hormone (TRH), which initiates TSH production and subsequent thyroid hormone formation. Normal thyroid function is governed by a tightly regulated feedback mechanism, whereby low levels of T3 and T4 signal to the pituitary to produce more TSH and to the thyroid to make more thyroid hormones. Conversely, excess levels of these hormones normally signal the pituitary and thyroid glands to decrease their production, so that levels of thyroid hormones remain adequate and not in excess.

Most thyroid diseases are the result of dysfunction within this elaborate regulatory mechanism, such that inappropriate levels of thyroid hormones are produced, thereby disrupting normal growth, metabolism and function in the body. In addition, thyroid hormone production requires adequate dietary intake of iodine. As a result, iodine deficiency is the most common underlying cause of thyroid disorders globally, with an estimated two billion individuals including 285.4 million children classified as iodine deficient by a WHO 2004 report.

Hypothyroidism

Hypothyroidism arises when there are inadequate levels of thyroid hormone available to ensure normal growth, metabolism and tissue function. It is the most common of thyroid diseases, preferentially affecting women, the elderly, and those with a family history of thyroid disease. Characterized largely by nonspecific clinical symptoms—including fatigue, mental depression, intolerance to cold, slight weight gain, dry skin and hair, and constipation—the diagnosis of hypothyroidism can be easily confirmed by thyroid function tests. These tests examine blood levels of TSH, T3 and T4, where low levels of T and T4 are diagnostic of hypothyroidism.

The most common cause of hypothyroidism is Hashimoto's disease, an autoimmune condition where the immune system attacks the thyroid gland, impairing its function. Other causes of hypothyroidism include iodine deficiency, treatment with radioactive iodine, or surgical removal of the thyroid gland (the latter two often done in the treatment of other thyroid disorders) as well as adverse consequences of certain medications, such as amiodarone.

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