Thyroid Blood Tests (cont.)
Ruchi Mathur, MD, FRCP(C)
Ruchi Mathur, MD, FRCP(C) is an Attending Physician with the Division of Endocrinology, Diabetes and Metabolism and Associate Director of Clinical Research, Recruitment and Phenotyping with the Center for Androgen Related Disorders, Department of Obstetrics and Gynecology at Cedars-Sinai Medical Center.
William C. Shiel Jr., MD, FACP, FACR
Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.
In this Article
- Introduction
- What are thyroid hormones?
- Thyroid hormone regulation -- the chain of command
- How is hypothyroidism diagnosed?
- How is hyperthyroidism diagnosed?
- Are there other tests of the thyroid gland?
How is hypothyroidism diagnosed?
A diagnosis of hypothyroidism can be suspected in patients with fatigue, cold intolerance, constipation, and dry, flaky skin. A blood test is needed to confirm the diagnosis.
When hypothyroidism is present, the blood levels of thyroid hormones can be measured directly and are usually decreased. However, in early hypothyroidism, the level of thyroid hormones (T3 and T4) may be normal. Therefore, the main tool for the detection of hyperthyroidism is the measurement of the TSH, the thyroid stimulating hormone. As mentioned earlier, TSH is secreted by the pituitary gland. If a decrease of thyroid hormone occurs, the pituitary gland reacts by producing more TSH and the blood TSH level increases in an attempt to encourage thyroid hormone production. This increase in TSH can actually precede the fall in thyroid hormones by months or years (see the section on Subclinical Hypothyroidism below). Thus, the measurement of TSH should be elevated in cases of hypothyroidism. However, there is one exception. If the decrease in thyroid hormone is actually due to a defect of the pituitary or hypothalamus, then the levels of TSH are abnormally low. As noted above, this kind of thyroid disease is known as "secondary" or "tertiary" hypothyroidism. A special test, known as the TRH test, can help distinguish if the disease is caused by a defect in the pituitary or the hypothalamus. This test requires an injection of the TRH hormone and is performed by an endocrinologist (hormone specialist).
How is hyperthyroidism diagnosed?
Hyperthyroidism can be suspected in patients with tremors, excessive sweating, smooth, velvety skin, fine hair, a rapid heart rate, and an enlarged thyroid gland. There may be puffiness around the eyes and a characteristic stare due to the elevation of the upper eyelids. Advanced symptoms are easily detected, but early symptoms, especially in the elderly, may be quite inconspicuous. In all cases, a blood test is needed to confirm the diagnosis.
The blood levels of thyroid hormones can be measured directly and are usually elevated with this disease. However, the main tool for detection of hyperthyroidism is measurement of the blood TSH level. As mentioned earlier, TSH is secreted by the pituitary gland. If an excess amount of thyroid hormone is present, TSH is " down-regulated" and the level of TSH falls in an attempt to control thyroid hormone production. Thus, the measurement of TSH should result in low or undetectable levels in cases of hyperthyroidism. However, there is one exception. If the excessive amount of thyroid hormone is actually due to a TSH secreting pituitary tumor, then the levels are abnormally high. This uncommon disease is known as "secondary hyperthyroidism."
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