![]() About 85% of patients have normal TSH values and do not require further testing.ĭecreased TSH values suggest primary hyperthyroidism, exogenous hyperthyroxinemia, or the effect of glucocorticoids. Linear changes in FT 4 concentration result in logarithmic changes in TSH secretion. TSH usually shifts out of the normal range sooner than thyroxine or FT 4. Testing begins with TSH, because it is the most sensitive indicator of thyroid metabolism in patients with normal pituitary gland function. The American Thyroid Association recommends the combined use of TSH and free T 4 as the most efficient combination of blood tests for diagnosis and follow-up of both ambulatory and hospitalized patients. The introduction of sensitive TSH (thyroid stimulating hormone) assays has transformed thyroid function testing from a thyroxine to a TSH based strategy. New methods for direct measurement of free T 4 has eliminated the need to estimate free hormone levels with T 3U and FTI. The product of these tests was reported as the free thyroxine index (FTI) or T 7. Traditionally, at least two thyroid function tests, thyroxine (T 4) and T 3 uptake (T 3U), were ordered on each patient. They account for 4% of all outpatient laboratory tests and 8% of laboratory charges. Thyroid function tests are among the most common laboratory tests. Serum T 4, FT 4, T 3, and FT 3 concentrations are elevated two to three times above the upper limit of normal and TSH is undetectable. Graves’s disease, an autoimmune disorder caused by antibody to TSH receptors, leads to uncontrolled release of T 4 and T 3, which suppress circulating TSH. The opposite condition, hyperthyroidism or thyrotoxicosis, is caused by too much circulating T 4. At this stage of hypothyroidism, TSH is elevated and T 4 and free T 4 are decreased. If this last attempt to restore homeostasis fails, the patient’s metabolic rate falls. The failing thyroid gland partially compensates by increasing the ratio of secreted T 3 to T 4. ![]() The pituitary secretes more TSH in response to decreasing circulating T 4 concentrations. ![]() In this condition, the thyroid fails to respond to TSH. Primary hypothyroidism is the most common thyroid disorder. In peripheral tissue, T 4 is converted to T 3, which is the active hormone that migrates into the target cell nuclei and binds to DNA. Only the free fraction is metabolically active. Most circulating T 3 and T 4 are protein bound. T 4 and T 3 are secreted in a ratio of 10:1. TSH stimulates the release of triiodothyronine (T 3) and (thyroxine) T 4 from thyroglobulin and their secretion by the thyroid. Thyrotropin releasing hormone (TRH), a hypothalamic peptide, stimulates the anterior pituitary to synthesize and secrete thyroid stimulating hormone (TSH). ![]()
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