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The TSH, in turn, stimulates the thyroid to produce thyroid hormone until levels in the blood return to normal. Thyroid hormone exerts negative feedback control over the hypothalamus as well as anterior pituitary, thus controlling the release of both TRH from hypothalamus and TSH from anterior pituitary gland.
Prior to the availability of sensitive TSH assays, thyrotropin releasing hormone or TRH stimulation tests were relied upon for confirming and assessing the degree of suppression in suspected hyperthyroidism. Typically, this stimulation test involves determining basal TSH levels and levels 15 to 30 minutes after an intravenous bolus of TRH ...
TSH may be useful to detect poor thyroid output and may reflect the state of thyroid hormones in the hypothalamic-pituitary-thyroid axis, but not the presence of hormones in other body tissues. [ 22 ] [ 79 ] [ 87 ] As a result, LT 4 monotherapy may not result in a "truly biochemically euthyroid state."
Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.
Stress suppresses TSH, [5] and alterations in thyroid hormone levels may arise in psychiatric illness. In major depressive disorder, an NTIS-like phenotype may be observed, with reduced T3 and increased rT3. T4 may be elevated, and TSH is usually normal, although TSH's normal circadian rhythm may be disrupted. [2]
The hypothalamic-pituitary-thyroid axis. TRH can be seen in green. TRH is synthesized within parvocellular neurons of the paraventricular nucleus of the hypothalamus. [2] It is translated as a 242-amino acid precursor polypeptide that contains 6 copies of the sequence -Gln-His-Pro-Gly-, with both ends of the sequence flanked by Lys-Arg or Arg-Arg sequences.
Any inappropriateness of measured values, for instance a low-normal TSH together with a low-normal T 4 may signal tertiary (central) disease and a TSH to TRH pathology. Elevated reverse T 3 (RT 3 ) together with low-normal TSH and low-normal T 3 , T 4 values, which is regarded as indicative for euthyroid sick syndrome, may also have to be ...
Free T4, total T3 or both are elevated and serum TSH is below normal in hyperthyroidism. If the hyperthyroidism is mild, only serum T3 may be elevated and serum TSH can be low or may not be detected in the blood. [14] Free T4 levels may also be tested in patients who have convincing symptoms of hyper- and hypothyroidism, despite a normal TSH.