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In overt primary hyperthyroidism, TSH levels are low and T 4 and T 3 levels are high. Subclinical hyperthyroidism is a milder form of hyperthyroidism characterized by low or undetectable serum TSH level, but with a normal serum free thyroxine level. [32]
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.
Thyroxine or tetraiodothyronine (T4) is produced by the thyroid gland. The term is sometimes used to refer to hyperthyroidism, but hyperthyroidism is a more general term. [2] When the level of thyroxine (T4) in the blood exceeds normal range, it can lead to symptoms such as irritability and unexplained weight loss. [3] Types include:
Free T4 and total T3 can be measured when hyperthyroidism is of high suspicion as it will improve the accuracy of the diagnosis. 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]
Text books have stated that diffusion is the main means of transport, [45] but recent studies indicate that monocarboxylate transporter (MCT) 8 and 10 play major roles in the efflux of the thyroid hormones from the thyroid cells. [32] [33]
The TSHr is expressed on the thyroid follicular cells of the thyroid gland (the cells that produce thyroid hormone), and the result of chronic stimulation is an abnormally high production of T3 and T4. This, in turn, causes the clinical symptoms of hyperthyroidism, and the enlargement of the thyroid gland visible as goiter.
Euthyroid sick syndrome (ESS) is a state of adaptation or dysregulation of thyrotropic feedback control [1] wherein the levels of T3 and/or T4 are abnormal, but the thyroid gland does not appear to be dysfunctional.
Thyroid hormone binding ratio (THBR) is a thyroid function test that measures the "uptake" of T3 or T4 tracer by thyroid-binding globulin (TBG) in a given serum sample. This provides an indirect and reciprocal estimate of the available binding sites on TBG within the sample.