TSH, or thyroid stimulating hormone, signals the thyroid to make and excrete thyroid hormone into the body, which effects the speed of metabolism. Hypothyroidism, which consists of low thyroid hormone, and hyperthyroidism, which consists of high thyroid hormone, can both lower TSH, depending on the cause. It is important to consult a physician for the diagnosis and treatment of thyroid conditions.
Thyroid Hormone Regulation
If thyroid hormones are low, TRH, thyrotropin releasing hormone, is released from a part of the brain called the hypothalamus. TRH travels to the pituitary in the skull, which releases TSH. TSH then travels to the thyroid gland in the neck and signals it to release thyroid hormone. Thyroid hormone then enters the bloodstream to affect all the cells of the body. It also binds to the TRH releasing cells of the hypothalamus and TSH releasing cells of the pituitary, inhibiting them from releasing TRH and TSH. This is called negative feedback. When thyroid hormone levels drop, TRH and TSH are once again released.
TSH Secretion and Measurement
TSH is secreted from pituitary in response to TRH and is inhibited in response to thyroid hormone. Changing the level of these factors changes the secretion of TSH and therefore its concentration in the blood. A TSH assay measures this concentration and is a powerful test for detecting the presence of thyroid disease.
Causes of Low TSH
Low TSH is caused by hyperthyroidism, secondary hypothyroidism and tertiary hypothyroidism. In hyperthyroidism, high levels of thyroid hormone signal the pituitary to make less TSH, so the level in the blood is lower. In addition, there will be low TRH. Hypothyroidism comes from several sources. In primary hypothyroidism, a problem with the thyroid gland causes it to not release thyroid hormone. In secondary hypothyroidism, a problem with the pituitary causes it to not release TSH, so its level is low in the blood. In tertiary hypothyroidism, a problem with the hypothalamus causes it to not release TRH, so TRH fails to stimulate the pituitary to release TSH. This is in contrast with primary hypothyroidism, in which the low thyroid hormone leads to increased levels of TSH.
Treatment
The treatment of a low TSH depends on the cause. In the case of hyperthyroidism, medicines are given to counter the negative effects of the excessive thyroid hormone and inhibit the synthesis of thyroid hormone by the thyroid gland. In addition, the thyroid can be removed surgically or by medicines, which then requires thyroid hormone replacement. For hyperthyroidism, a doctor will prescribe thyroid hormone pills.
References
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci et al.; 17th Ed 2008
- "Robbins and Cotran Pathologic Basis of Disease"; Vinay Kumar et al.; 8th Ed 2009
- "Physiology"; Linda S.Costanzo; 4th Ed 2008
- "Current Medical Diagnosis and Treatment 2011"; Stephen McPhee, Maxine Papadakis, and Michael W. Rabow; 50th Ed 2010
