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TSH, also known as Thyroid Stimulating Hormone or Thyroid-stimulating hormone, is a hormone produced by the pituitary gland. The main function of TSH is to regulate activity of the thyroid gland and the hormones it produces, including T3 (triiodothyronine) and T4 (thyroxine).
The body has intricate feedback mechanisms that alert the pituitary when the body needs more or less thyroid activity. When the pituitary gets a signal that the body needs more thyroid activity, it releases more TSH which sends a signal to the thyroid gland that tells it to increase the production of thyroid hormone. If the pituitary gets a signal that the body needs less thyroid activity, it decreases its production of TSH in an effort to reduce the overall activity of the thyroid gland.
An understanding of how the thyroid works is helpful in making sense of what an abnormal TSH level means. When the TSH blood test level is high, this suggests that excessive TSH is being produced in an attempt to stimulate the thyroid gland. This frequently happens because the pituitary gland is getting a signal from the body that it needs more thyroid hormone. As a result, a high TSH level suggests that one has an underactive thyroid (i.e. low thyroid activity) which can typically be better assessed with a more comprehensive hypothyroidism test panel.
Conversely, a low TSH level suggests that the thyroid might be overactive (i.e. high thyroid activity). In this situation, the pituitary gland is receiving feedback from the body that the thyroid is too active and, as a result, needs less stimulation. To reduce thyroid stimulation, the pituitary gland cuts back on the production of TSH with the intention of reducing the production of thyroid hormone by the thyroid gland. While a TSH test result can help screen for an overactive thyroid, more detailed investigation of high thyroid gland activity can be done with the hyperthyroidism test panel.
As a result, the measured level of TSH using the TSH blood test is useful because it serves as an indicator as to whether the thyroid gland is doing its job. Because it gives a snapshot of the overall health of the thyroid gland, the TSH test is frequently used as an initial screening test for overall thyroid health. Normally, TSH levels will vary throughout the day as the body turns on and off processes in connections with its daily cycles. As with most lab tests, measuring one's TSH levels over time is helpful because it can help identify abnormal trends that are specific to one's body.
Last updated August 11, 2017