The thyroid gland produces two hormones, known as T3 and T4, which regulate the body's metabolism. The thyroid gland's activity is itself controlled by another hormone, known as TSH or thyroid stimulating hormone. Doctors often check the levels of this hormone as part of a routine thyroid panel blood test; low TSH levels can be caused by a variety of disorders.
Significance
TSH is made by the pituitary gland, the American Thyroid Association explains. The pituitary gland makes more TSH if it detects abnormally low levels of T4 in the body, as TSH stimulates the production of T4. TSH production continues until adequate T4 levels are reached. This is an example of a feedback mechanism, in which low levels of one hormone causes an increase in the production of a second hormone which stimulates production of the first hormone. Normal TSH levels are between 0.4 and 4 mIU/L.
Symptoms
Low levels of TSH can result in the symptoms of hypothyroidism or hyperthyroidism, depending on the cause of the low TSH levels. Patients with hypothyroidism may experience constant fatigue, weight gain and are often are sensitive to cold. Other symptoms of hypothyroidism include depression and constipation. Patients who have hyperthyroidism may have an increased appetite, have unexpected weight loss, be intolerant to heat, and sweat more than usual.
Causes
Abnormally low levels of thyroid stimulating hormones can be caused by problems with the pituitary, with the thyroid or due to the use of certain medications. Hyperthyroidism can result in depressed levels of TSH, Lab Tests Online explains, because the elevated T4 levels will suppress thyroid stimulating hormone production. Low TSH levels may also occur in patients who are receiving synthetic thyroid hormone if their dose is too high. In some cases, low TSH levels are the result of damage to the pituitary gland, which results in the inability to produce adequate amounts of TSH. Finally, TSH production can be inhibited by the use of certain medications, such as dopamine agonists, bexarotene, somatostatin analogues and glucocorticoids, MedLinePlus reports.
Additional Tests
Interpretation of low thyroid stimulating hormone levels requires further analysis. The use of drugs which can interfere with TSH production should be ruled out. Low TSH levels combined with low thyroid hormone levels indicate a problem with pituitary function. If the levels of the thyroid hormones T4 and T3 are abnormally high, then hyperthyroidism is the cause of the decreased TSH levels and the underlying cause of the hyperthyroidism must be determined. Hyperthyroidism can be the result of an infection, thyroid cancer, or an autoimmune disease known as Hashimoto's thyroiditis.
Treatment
Patients who have a pituitary disorder that blocks TSH production may need to take synthetic forms of thyroid hormone to prevent hypothyroidism. because TSH has no other role in the body other than to regulate thyroid hormone production, this will effectively correct the problem. Patients who have hyperthyroidism can take medications which reduce the production of thyroid hormone, the National Institute of Diabetes and Digestive and Kidney Disorders explains. The activity of the thyroid hormone can also be decreased using radioactive iodine, which can destroy abnormally functioning thyroid tissue. Finally, the thyroid can be surgically removed in situations where no other treatment works well.
