TSH Blood Test - More Information

The TSH test, also known as a TSH lab test, TSH level test and thyroid-stimulating hormone test, measures the TSH blood level.

TSH, or thyroid-stimulating hormone, is a hormone produced by the pituitary gland. Made by special cells called thyrotrope cells in the anterior part of the pituitary, TSH plays a vital role in regulating the activity of the thyroid gland. To do so, TSH binds to receptors found on the thyroid gland's follicular cells.

Regulating the thyroid gland is important because the thyroid produces several hormones which can stimulate or shut down metabolic processes in the body. The most important hormones produced by the thyroid are T3, or triiodothyronine, and T4, or thyroxine, and stimulation by TSH can turn on or off their production. This is why measuring T3 levels through the total T3 test and T4 levels through the total T4 test is sometimes done in conjunction with TSH testing.

TSH is produced by the pituitary in response to signals from the body. The body has intricate feedback mechanisms to alert the pituitary when the body needs more or less thyroid hormone to fuel metabolic processes. When the pituitary gets a signal that the body needs more thyroid activity, it releases more TSH to tell the thyroid gland to produce more thyroid hormone. If the pituitary gets a signal that the body needs less thyroid activity, it decreases its production of TSH to tell the thyroid gland to slow down its thyroid hormone production. To ensure tight, timely regulation of the thyroid gland, circulating TSH in the blood is broken down quickly by the body.

For TSH to activate the thyroid, the thyroid gland needs to be working properly. The thyroid receptors on the gland must be able to interact with circulating thyroid-stimulating hormone and enough receptors need to be available for TSH to bind to them. Signals from the thyroid receptors to the thyroid hormone production processes must be transmitted properly. The thyroid hormone production machinery needs to be able to respond to signals to produce adequate amounts of T3 and T4 once they are told to do so.

This understanding of how the thyroid gland works is helpful in making sense of what an abnormal TSH level means. When the TSH blood level is high, this suggests that the pituitary is trying to stimulate the thyroid gland. This frequently happens because the pituitary gland is getting a signal that the body needs more thyroid hormone for various processes. Clinically, a high TSH level often suggests that one has an underactive thyroid (i.e. low thyroid activity or hypothyroidism). In settings where hypothyroidism is a concern, TSH testing is often done with other lab tests as part of the more comprehensive hypothyroidism test panel.

Conversely, a low TSH level suggests that the thyroid might be overactive (i.e. high thyroid activity or hyperthyroidism). 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.

Getting a TSH test near you should be relatively straightforward as the TSH blood test is a common lab test and every standard Quest Diagnostics lab location should have the equipment needed to perform it. On the day of your test, a small amount of your blood will be drawn into a vial and the specimen will be run in a machine. TSH lab results come back relatively quickly and a reference range will be provided on the lab report against which you can compare your results. On the TSH sample lab report provided above, the TSH test result reported is 0.03 mIU/L which is considered low when compared against the provided normal range of 0.40-4.50 mIU/L. Low TSH levels in this case might indicated hyperthyroidism and should be interpreted in the context of clinical symptoms and possibly with more advanced thyroid lab tests.

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.