Did your doctor say you have a low TSH level? A TSH level is measured by drawing a tube of blood and sending it to the lab where the level of TSH in the blood can be quantified. What does a low TSH level mean?
What is TSH?
TSH is a hormone produced by the pituitary gland located in the brain. When TSH is released by the pituitary, it sends a signal to the thyroid gland to produce more thyroid hormone – a group of two hormones that regulate the body’s metabolic rate. When enough thyroid hormone has been produced, the two thyroid hormones, T3 and T4 send a message back to the pituitary with instructions to stop releasing TSH – a process called feedback inhibition. In this way, the body can tightly regulate thyroid hormone production so too much or too little isn’t produced. Unfortunately, sometimes this system goes awry.
What Does a Low TSH Mean?
A low TSH test means one of two things. TSH levels may be low because the pituitary gland isn’t producing enough of it, or, more commonly, because the thyroid gland is producing too much thyroid hormone, which signals the pituitary to cut back on TSH production. Remember feedback inhibition?
Why Does a Doctor Check a TSH Level?
A doctor draws a TSH to check for thyroid disease. At the same time TSH is checked, levels of thyroid hormones, T3 and T4 are determined to see if they’re abnormally high or low. Knowing the levels of these three hormones makes the diagnosis of an over or under-active thyroid easier.
Doctors often order these tests when a patient complains of symptoms consistent with thyroid disease or when they feel an enlargement or nodule in the thyroid gland. TSH levels are also monitored in patients receiving medications for an over or under-active thyroid gland – to make sure they’re taking the right amount.
What Does a Low TSH Mean?
If you have a low TSH level and high levels of the thyroid hormones, T3 and T4, this usually indicates an overactive thyroid, a condition called hyperthyroidism. On the other hand, if TSH levels are low and T3 and T4 are also low, it may mean that the pituitary gland is damaged or diseased.
The pituitary can be damaged as a result of a head injury or, sometimes, from a tumor. Since the pituitary produces eight different hormones, blood tests can be done to see if these hormones also low, which would be further indicative of a pituitary problem. Brain imaging studies may also be needed to rule out the possibility of a pituitary tumor if TSH and thyroid hormone levels are low.
What Does a Low TSH Mean: The Bottom Line?
A low TSH level usually means the thyroid gland is overactive, but it can also indicate a problem with the pituitary gland, especially if T3 or T4 is also low. Ask your doctor about your TSH, T3, and T4 levels – and if they were outside the normal range.
References:
Merck Manual. Eighteenth Edition. 2006.
