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Quick GuideThyroid Anatomy, Symptoms, and Disorders
Thyroid hormone regulation: the chain of command
The thyroid itself is regulated by another gland that is located in the brain, called the pituitary. In turn, the pituitary is regulated in part by the thyroid (via a "feedback" effect of thyroid hormone on the pituitary gland) and by another gland called the hypothalamus.
The hypothalamus releases a hormone called thyrotropin releasing hormone (TRH), which sends a signal to the pituitary to release thyroid stimulating hormone (TSH). In turn, TSH sends a signal to the thyroid to release thyroid hormones. If a disruption occurs at any of these levels, a defect in thyroid hormone production may result in a deficiency of thyroid hormone (hypothyroidism).
Hypothalamus - TRH
Pituitary - TSH
Thyroid - T4 and T3
The rate of thyroid hormone production is controlled by the pituitary gland. If there is an insufficient amount of thyroid hormone circulating in the body to allow for normal functioning, the release of TSH is increased by the pituitary gland in an attempt to stimulate more thyroid hormone production. In contrast, when there is an excessive amount of circulating thyroid hormone, TSH levels fall as the pituitary attempts to decrease the production of thyroid hormone. In persons with hypothyroidism (thyroid hormone production is below normal), there is a continuously decreased level of circulating thyroid hormones. In persons with hyperthyroidism (thyroid hormone production is above normal), there is a continuously elevated level of circulating thyroid hormones.
How is hypothyroidism diagnosed?
When hypothyroidism is present, the blood levels of thyroid hormones can be measured directly and are usually decreased. However, in early hypothyroidism, the level of thyroid hormones (T3 and T4) may be normal. Therefore, the main tool for the detection of hyperthyroidism is the measurement of the TSH, the thyroid stimulating hormone. As mentioned earlier, TSH is secreted by the pituitary gland. If a decrease of thyroid hormone occurs, the pituitary gland reacts by producing more TSH and the blood TSH level increases in an attempt to encourage thyroid hormone production. This increase in TSH can actually precede the fall in thyroid hormones by months or years (see the section on Subclinical Hypothyroidism below). Thus, the measurement of TSH should be elevated in cases of hypothyroidism. However, there is one exception. If the decrease in thyroid hormone is actually due to a defect of the pituitary or hypothalamus, then the levels of TSH are abnormally low. As noted above, this kind of thyroid disease is known as "secondary" or "tertiary" hypothyroidism. A special test, known as the TRH test, can help distinguish if the disease is caused by a defect in the pituitary or the hypothalamus. This test requires an injection of the TRH hormone and is performed by an endocrinologist (hormone specialist).