What Affects TSH Levels?

TSH, or thyroid stimulating hormone, is a crucial component of the precise control mechanism that the body uses to regulate the thyroid and, by extension, metabolism.  Understanding how the body regulates this hormone in normal health and how its levels change in various disorders is important for understanding why thyroid disease occurs and how doctors use TSH levels to test for various thyroid problems.

Normal

In normal health TSH release is controlled by the hypothalamus, the link between the central nervous system and the endocrine system. The hypothalamus secretes TRH (thyrotropin releasing hormone), signaling the pituitary to release TSH (thyroid stimulating hormone).  This triggers the thyroid gland to synthesize and release thyroid hormones. When levels of thyroid hormones in the blood rise above a normal threshold, they inhibit the release of TRH from the hypothalamus. This in turn decreases the release of TSH from the pituitary, causing release of thyroid hormones to decrease. Through this negative feedback mechanism, the body maintains thyroid hormones at a relatively constant level.

Disease

Diseases of the hypothalamus, pituitary and thyroid gland can affect normal control of TSH and cause abnormal hormone levels. If the thyroid is diseased and cannot produce thyroid hormones, TSH levels will be high in the blood as the hypothalamus and pituitary respond to the body's need for higher thyroid hormone levels. If, on the other hand, thyroid disease is causing an overproduction of thyroid hormones without regard to pituitary signaling, TSH levels will be low. In cases where the pituitary is diseased or damaged and cannot produce sufficient TSH, levels of this hormone will also be low. A TSH secreting tumor of the pituitary, on the other hand, will cause an increase in TSH levels. Although rare, it is possible for tumors or disorders of the hypothalamus to cause increased or decreased TRH levels which will also affect TSH levels.