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Hypothyroidism

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of hypothyroidism.

Alternative Names

Autoimmune thyroiditis; Hashimoto's thyroiditis

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

These hormones can also alter the actions of other hormones and drugs.

Iodide and Thyroid Hormone Production

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:

• 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, they typically bind to substances called thyroid hormone transport proteins, after which they become inactive.
• Thyrotropin. Thyrotropin (also called thyroid-stimulating hormone or TSH ) is another very important hormone in the process. Secreted by the pituitary gland, this hormone directly influences the 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, which monitors thyrotropin levels.

Hypothyroidism

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.

Subclinical Hypothyroidism

Subclinical 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.

For instance, mildly underactive thyroid does not progress to the full-blown disorder in most people. Experts estimate that each year approximately 2 - 5% of people with mildly underactive thyroid will go on to develop overt hypothyroidism. Other factors associated with a higher risk include being an older woman (up to 20% of women over age 60 have subclinical hypothyroidism), having a goiter (enlarged thyroid gland) or thyroid antibodies, or harboring immune factors that suggest an autoimmune condition.

Mildly underactive thyroid is determined on the basis of the TSH laboratory blood tests. According to a 2004 consensus statement from the American Thyroid Association, the American Association of Clinical Endocrinologists, and the Endocrine Society, the normal range of TSH concentration falls between 0.45 and 4.5 mU/L. Patients with mildly underactive thyroid have TSH levels between 4.5 mU/L and 10mU/L. Patients with levels greater than 10mU/L are considered to have overt hypothyroidism and should be treated with medication.

For patients in the mildly underactive thyroid range, treatment decisions are less clear. The consensus committee recommended against routine treatment for patients with mildly underactive thyroid , but did suggest repeat screenings of thyroid function every 6 to 12 months to detect any changes in TSH levels. However, these are general guidelines, and individual cases and risk factors may differ. Patients should discuss with their doctor the course of action that is most appropriate for them.

• Review Date: 4/19/2006
• Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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