Hypothyroidism - Introduction

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Click the icon to see an image of the pituitary gland.

Click the icon to see an image of the pituitary gland and TSH.

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 gland 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.

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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.

• Patients with mildly underactive (subclinical) thyroid have TSH levels of 4.5 - 10mU/L.
• Patients with levels greater than 10mU/L are considered to have overt hypothyroidism and should be treated with medication.

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 doctors because it is not clear how to manage this condition.

Mildly underactive thyroid does not progress to the full-blown disorder in most people. Each year, about 2 - 5% of people with subclinical thyroid go on to develop overt hypothyroidism. Other factors associated with a higher risk of developing clinical hypothyroidism 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.

Review Date: 05/03/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org).