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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.
Both excess and deficiency of thyroxine can cause disorders. Hyperthyroidism (an example is Graves' disease) is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.
Hypothyroidism is diagnosed by looking at the free thyroxine (T4) levels in people with elevated TSH levels, and comparing the ratio between them. People with high TSH and low T4 get a diagnosis ...
When circulating in the body, T3 and T4 are bound to transport proteins. Only a small fraction of the circulating thyroid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3 and T4, or total T3 and T4, which takes into consideration the free hormones in addition to the protein-bound hormones.
Thyroxine, also known as T 4, is a hormone produced by the thyroid gland. It is the primary form of thyroid hormone found in the blood and acts as a prohormone of the more active thyroid hormone, triiodothyronine (T 3 ). [ 1 ]
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. The results are then reported as a ratio to normal serum.
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] Although the evidence for doing so is not definitive, treatment of elderly persons having subclinical hyperthyroidism could reduce the number of cases of atrial fibrillation. [33]
However, as the two thyroid hormones travel bound to other molecules, and it is the "free" component that is biologically active, free T 3 and free T 4 levels can be measured. [84] T 3 is preferred, because in hypothyroidism T 3 levels may be normal. [84] The ratio of bound to unbound thyroid hormones is known as the thyroid hormone binding ...