Thyroid function tests are blood tests that show how well the thyroid gland in the neck is producing thyroid hormone, and how much it has to be stimulated to do so.
What is TSH?
Thyroid stimulating hormone (TSH) is produced in the pituitary gland, located in the head. When the TSH is high, it can indicate that the thyroid gland is having difficulty producing enough thyroid hormone and therefore needs abnormally high levels of TSH to make it work. Low TSH levels can indicate high thyroid hormone levels turning off that particular pituitary function. A thyroid screening test generally consists of only the TSH because it is a very sensitive marker for hypothyroidism and hyperthyroidism. If your TSH is abnormal, a free T4 can be ordered separately, or free T4 may be a “reflex” test, that is, it will automatically be ordered and tested for if you have an abnormal TSH result.
What is free T4?
A free T4 test specifies the amount of thyroid hormone in the blood. A low level is known as hypothyroidism. This can result in fatigue, weight gain, cold intolerance, hair thinning, poor concentration, constipation, and dry skin. The opposite condition is hyperthyroidism, which can result in heat intolerance, fast heart beat, nervousness, and unexplained weight loss.
What are normal ranges for thyroid function test results?
Normal TSH: 0.5 to 5.0 milli-international units per liter
Normal T4: 5.0 to 12.0 micrograms/dL