If your healthcare provider suspects that you have Graves’ disease, or some other form of hyperthyroidism, the first test that they are likely to order is a TSH level. This test is very useful because the pituitary gland, which makes TSH, is an excellent “thermostat” and is very “clued in” to the activity of the thyroid gland.
If the thyroid gland has become overactive from stimulation by the immune system, the pituitary gland will recognize that there is too much thyroid hormone in the blood and it will stop making TSH. Thus, the level of TSH in the blood will be very low and usually unmeasurable.
Your doctor may tell you that your TSH level is suppressed. This situation is exactly the same as if the heater in your house got stuck in the “on” position and the house heats up to 90 degrees. Your thermostat would shut itself off, knowing that it was too hot in the house.
Because the pituitary gland is such a sensitive thermostat, it completely stops making even when the thyroid gland is only very mildly overactive. Therefore, in a person with Graves’ disease and hyperthyroidism, additional blood tests are needed to determine the severity of the condition.
The best tests for this are to measure the total T4 and T3 levels in the blood, since the more overactive the thyroid gland is, the higher these levels will be. Normally, the total T4 level in the blood is about 5-12 (micrograms per deciliter), and the total T3 level is about 75-180 (nanograms per deciliter). In mild cases of hyperthyroidism the total T4 and total T3 may be only slightly above normal, whereas in severe cases, the T4 may be over 20 and the T3 over 500.
Some laboratories measure what is called the “free T4″ level instead of the total T4 test. The normal value for the free T4 is different from the total T4, but the information learned from the test is the same — in hyperthyroidism, the free T4 value is elevated.
The combination of a suppressed TSH and elevations in the T4 and T3 levels indicate that a person is hyperthyroid, and the levels allow the physician to determine how severe the disease is. An important question then becomes whether the hyperthyroidism is due to Graves’ disease or some other form of thyroid disease.
This is important because different forms of hyperthyroidism are treated in different ways. Usually the physician can determine the type of hyperthyroidism based on:
a careful assessment of the patients symptoms. (Patients with Graves’ disease usually have had symptoms for several months, whereas in other forms of hyperthyroidism, the symptoms may have started only a few days or weeks ago.)
the age of the patient. (In patients under age 50, hyperthyroidism is most commonly due to Graves’ disease.)
the size and consistency of the thyroid gland. (Patients with Graves’ disease have enlarged and smooth thyroid glands that are usually not tender, whereas in other forms of hyperthyroidism the thyroid gland may be “lumpy” or it may be painful to the touch.)
The presence of ophthalmopathy or a thyroid bruit. (Only patients with Graves’ disease have these conditions.)
If your physician is not sure that your hyperthyroidism is due to Graves’ disease, she or he may order other tests.
These might include a radioactive iodine uptake test, a scan of the thyroid gland, or the measurement of antibody levels in the blood. However, these are relatively expensive tests and usually are not necessary for diagnosis.