Thyroid stimulating hormone(TSH) is a hormone secreted by adenohypophysis to promote the growth and function of the thyroid gland. The changes of TSH is noted prior to the T4 and T3 for months or years, as is of great value for the diagnosis of thyroid dysfunction and lesions.

Clinical Significance

High Levels of TSH TSH levels typically fall between 0.4 and 4.0 milliunits per liter (mU/L), according to the American Thyroid Association. Ranges between laboratories will vary with the upper limit generally being between 4 to 5. If your level is higher than this, chances are you have an underactive thyroid. In general, T3 and T4 levels increase in pregnancy and TSH levels decrease.

Low Levels of TSH
It's also possible that the test reading comes back showing lower than normal levels of TSH and an overactive thyroid. This could be caused by:
Graves’ disease (your body’s immune system attacks the thyroid)

• Too much iodine in your body
• Too much thyroid hormone medication
• Too much of a natural supplement that contains the thyroid hormone