Hypothyroidism, a thyroid hormone deficiency, affects as many as 10 percent of women and 6 percent of men over age 65, the Merck Manuals Online Medical Library reports, but can also occur in younger people. The disease is easier to recognize in the young than in the elderly, the Merck website adds. Hypothyroidism can occur as a primary or secondary disease, with primary disease, the more common, caused by problems in the thyroid itself and secondary disease caused by problems with the hypothalamus or pituitary gland. Blood tests can diagnose a hypothyroid condition.
Thyroid Stimulating Hormone
Thyroid stimulating hormone, or TSH, is produced by the pituitary gland and plays an important part in the loop feedback mechanism that controls the thyroid. Release of TSH signals the thyroid to manufacture two thyroid hormones, T3 and T4. When the levels of thyroid hormone rise, the release of TSH is shut off. When a person has primary hypothyroid disease, the thyroid hormone levels in a blood sample are low because of disease in the thyroid. In response to low thyroid hormone levels, the pituitary releases more TSH, trying to force the thyroid to produce more thyroid hormone. In primary hypothyroidism, TSH levels rise. A rise in serum TSH is usually the first sign of diseased thyroid gland, endocrinologist James Norman, M.D., reports on the website endocrineweb. If secondary hypothyroidism results from damage to the pituitary gland, TSH levels remain low.
T4 Levels
Blood levels of T4, the active form of thyroid hormone, fall when a person has either primary or secondary hypothyroidism, the Merck Manuals Online Medical Library reports. In primary hypothyroidism, they fall because thyroid damage prevents hormone production. In secondary disease they fall because the pituitary isn't producing TSH, wich stimulates T4 production.
Thyroid Autoantibodies
Hashimoto's thyroiditis, the most common type of thyroid disease, can cause hypothyroidism if thyroid hormone levels can't keep up with the body's needs, the American Association of Clinical Endocrinologists states. Hashimoto's disease, an autoimmune disease in which autoantibodies attack the thyroid gland cells, can sometimes be diagnosed through blood tests for antithyroid antibodies.
Radioactive Iodine Uptake Scan
An iodine uptake scan measures the amount of iodine absorbed into the thyroid. The person undergoing the test takes a drink or pill containing radioactive iodine. Between six and 24 hours later, the amount of iodine taken up into the thyroid is measured. A hypoactive--or underactive--thyroid takes up less iodine than one that is normally active or hyperactive. The radioactive iodine is excreted in urine. While no harmful effects have been documented, pregnant and breastfeeding women should not undergo the test, the National Institutes of Health states.
