The thyroid, a small gland shaped like a tiny bow tie, rests under the skin just below the Adam’s apple. An underactive thyroid—inadequate production of the chemical that regulate’s the body’s metabolism—has several causes. The tests used to diagnose underactive thyroid establish its presence and help the doctor determine the cause. Underactive thyroid can be corrected with treatment, which must be taken for a lifetime.
Causes
Many conditions cause underactive thyroid. The most common cause in the United States is Hashimoto’s disease—a slowly progressing autoimmune disease that ultimately destroys the thyroid gland--reports MayoClinic.com. Rarely, inherited disorders prevent the thyroid from producing enough hormones. Surgical removal of all or part of the thyroid and exposure to radiation, either as treatment or atomic energy, also cause underactive thyroid.
Initial Testing
Usually, a simple blood test for levels of thyroid-stimulating hormone, TSH, is all that is needed to diagnose underactive thyroid, explains The Merck Manual. TSH is produced by the pituitary gland. The TSH levels are higher when the thyroid gland is less active because the pituitary gland is trying to stimulate the gland to produce more thyroid hormone, explains MayoClinic.com. Because mild hypothyroidism is common in older people, experts recommend that people over age 55 get a TSH test every two years, reports Merck.
Tests for Thyroid Hormones
Doctors often follow the TSH test with tests to determine how much hormone is being produced by the thyroid gland, and the pituitary gland, which influences the thyroid, reports MayoClinic.com. The two thyroid hormones measured are T4, thyroxine, the major thyroid hormone, and T3, triiodothyronine, an active hormone that regulates metabolism, explains the Merck Manual.
Anibody Tests
If autoimmune disease, especially Hashimoto’s disease, is suspected, doctors test for abnormal antibodies, explains MayoClinic.com. If autoimmune disease is present, antibodies against thyroid peroxidase, an enzyme in the thyroid gland that helps produce thyroid hormone, are detected.
Thyroid Scan
Thyroid scans produce detailed information about the size and problem in the thyroid, but they are expensive, invasive and time-consuming, reports MayoClinic.com. Scans are used if a doctor feels a nodule in the thyroid, reports the Merck Manual. They can tell if a specific area is underactive compared with the rest of the gland.
