If you fatigue easily, are sensitive to cold and have gained weight for no apparent reason, your doctor may recommend testing to see if you have low thyroid function. A condition called hypothyroidism can cause all of these symptoms and many more. Preliminary testing for low thyroid function involves a simple blood test. But if you've never had this type of testing done, you might wonder about the letters and numbers on your lab slip. Here's a simple guide to what your doctor is looking for when viewing your lab results.
What Does the Thyroid Do?
The thyroid is a small gland in the front of the neck composed of two lobes that wrap around the trachea and a band of tissue called the isthmus. The thyroid's role is to produce a hormone that regulates your metabolism--not only how quickly you lose weight, but your heart rate, blood pressure and body temperature. But the pituitary gland in the brain has a big role as well; if it detects that there's not enough thyroid hormone in the body, it dispatches its own thyroid stimulating hormone (TSH) to the thyroid gland. Initial testing for low thyroid function determines how much TSH and thyroid hormone is in your blood.
FT4 Levels
Although used in singular tense, the thyroid hormone is actually made of two hormones: thyroxine (T4) and triiodothyronine (T3). T4 is produced by the thyroid gland; some of this T4 is converted in organs such as the liver and the brain and becomes T3. Free T4 (FT4) is T4 that enters specific tissues of the body directly. Therefore, measuring the amount of FT4 in the blood is an effective measure of determining low thyroid function. FT4 levels that are lower than normal can be indicative of hypothyroidism.
TSH Levels
In conjunction with FT4 levels, TSH levels are also checked to see how much hormone is being released by your pituitary gland. When thyroid function is low, the pituitary gland continues to dispatch TSH until it's satisfied that the level of T4 in the body is normal, and then the pituitary shuts off. High levels of TSH may indicate that the pituitary gland is working overtime to no avail. High TSH levels combined with low FT4 levels indicate low thyroid function.
What About T3?
If your thyroid function is low enough to cause hypothyroidism, you might still have a normal T3 level, as this is often the last hormone to show fluctuation. Even someone who is in the extreme range of hypothyroidism can have high TSH levels and low FT4 levels but perfectly normal T3. So don't be surprised if your initial lab work only entails a TSH and FT4 test. T3 tests are most often used to determine if the thyroid is overactive.
Other Ways of Assessing Low Thyroid Function
An assessment of FT4 and TSH levels may tell your doctor if you have low thyroid function, but it won't explain the underlying cause. Many cases of hypothyroidism are caused by an autoimmune disorder called Hashimoto's disease. A thyroid antibody test can determine whether certain antibodies are present in the blood that are related to this disorder. A radioactive iodine uptake (RAIU) test involves having you swallow a small amount of radioactive iodine to determine how much of it the thyroid takes in. A low RAIU is noted when the thyroid gland is underactive. Additionally, a scan of the actual gland itself can be obtained to detect any abnormalities.
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