Your doctor can diagnose hyperthyroidism and hypothyroidism by testing the levels of thyroid hormones in your blood. Doctors measure hormones secreted by the thyroid itself, as well as thyroid-stimulating hormone (TSH), a chemical released by the pituitary gland that triggers hormone production in the thyroid.
When you are hypothyroid, higher quantities of TSH are circulating in your blood as your body attempts to increase production of thyroid hormones. The reverse is true with hyperthyroidism, in which TSH levels are below normal and circulating thyroid-hormone levels are high.
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To identify the cause of hyperthyroidism, doctors often use radioactive iodide uptake tests, which track the amount of iodide absorbed by the thyroid gland. Iodide, obtained from the foods we eat, is a key ingredient in the manufacture of thyroid hormone, so the amount of iodide the thyroid absorbs is a reliable indicator of how much hormone the gland is producing. For this test, the doctor places an instrument over your neck to measure how much background radioactivity there is. Then, you must swallow a small amount of radioactive iodide in liquid or capsule form. After a predetermined time (usually 4-6 hours and at 24 hours), the doctor again places an instrument over your neck to measure how much of the radioactive iodide has gathered in your thyroid.
If the test suggests that the gland is collecting excessive amounts of iodide, the doctor may then conduct a radioactive iodide uptake scan. In this test, the doctor uses a special film to create a picture that shows the exact location of the radioactive iodide in your thyroid gland. The scan will reveal, for example, if the iodide is collecting in nodules, indicating that the nodules are responsible for the excess hormone production. If the scan shows that the iodide is spread equally throughout the tissue, the whole thyroid is involved in the excess production.
Nodules that appear suddenly are typically fluid-filled cysts and are often benign. They can be evaluated with a noninvasive ultrasound exam. If blood tests indicate that the nodules are producing excess thyroid hormone, your doctor may treat you for hyperthyroidism.