The thyroid is a butterfly-shaped gland located at the front of the windpipe at the base of the throat. It controls metabolism (the rate at which the body converts food and oxygen into energy) by producing hormones called thyroxine (T4) and triiodothyronine (T3). The body also converts T4 into the more biologically powerful and rapidly acting T3. Levels of these hormones influence heart rate, body temperature, alertness, mood and many other functions. The thyroid is controlled by the pituitary gland, located deep within the brain. The pituitary regulates the thyroid, working much like a thermostat by releasing or withholding thyroid-stimulating hormone (TSH), depending on blood levels of T4 and T3. For example, when blood levels of thyroid hormones are low, the pituitary secretes TSH, which signals the thyroid to increase hormone production. On the other hand, when blood levels of T4 and T3 are high, the pituitary restricts the output of TSH, which signals the thyroid to slow down hormone production. When any part of this process malfunctions, the thyroid can produce too little hormone (a condition called hypothyroidism) or too much hormone (hyperthyroidism). Thyroid disorders are not always obvious since many symptoms of an underactive thyroid, such as weight gain and fatigue, are nonspecific and may be attributed to aging, menopause or depression. And symptoms of hyperthyroidism, like nervousness, insomnia and heart palpitations, may be attributed to anxiety. Consequently, thyroid disease is grossly underdiagnosed. Last Updated: 2/6/2003
The Johns Hopkins University 1996-2003. All rights reserved. This information is not intended to provide advice on personal medical matters, nor is it intended to be a substitute for consultation.
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