According to About.com, low levels of TSH, or thyroid-stimulating hormone, occur when the pituitary gland shuts down TSH production due to high levels of thyroid hormone in the blood. The low levels of TSH are the pituitary gland's attempt to keep thyroid hormone levels normal within the body.
Web MD explains that low TSH levels are also caused by an overactive thyroid gland due to Graves' disease or a toxic nodule, damage to the pituitary gland, taking too much thyroid medicine or early pregnancy. Levels of TSH under 0.4 microunits per milliliter are considered low. Low TSH levels are established through a TSH-level blood test, which takes two to three days to process.