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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.
Screening for thyroid disease in patients without symptoms is a debated topic although commonly practiced in the United States. [8] If dysfunction of the thyroid is suspected, laboratory tests can help support or rule out thyroid disease. Initial blood tests often include thyroid-stimulating hormone (TSH) and free thyroxine (T4). [9]
Further information: Thyroid function tests. Triiodothyronine (T 3) and thyroxine (T 4) can be measured as free T 3 and free T 4, which are indicators of their activities in the body. [73] They can also be measured as total T 3 and total T 4, which depend on the amount that is bound to thyroxine-binding globulin (TBG). [73]
[7] [20] [21] [22] Generally the levels of free T3 will be lowered, followed by the lowering of free T4 in more severe disease. Several studies described elevated concentrations of 3,5-T2, an active thyroid hormone, in NTIS.
Thyroid function testing frequently reveals decreased thyroid stimulating hormone (TSH) and increased serum levels of triiodothyronine (T3) and thyroxine (T4) during the acute phase of the disease. [2] The intrathyroidal T3 and T4 levels are often reflected by the T3 to T4 ratio, which is typically less than 20 (ng/dL divided by μg/dL). [22]
The test is performed by taking a blood sample, to which an excess of radioactive exogenous T 3 is added, followed by a resin that also binds T 3. A fraction of the radioactive T 3 binds to sites on TBG not already occupied by endogenous thyroid hormone, and the remainder binds to the resin. The amount of labeled hormones bound to the resin is ...
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 pituitary gland secretes thyrotropin (TSH; Thyroid Stimulating Hormone) that stimulates the thyroid to secrete thyroxine (T4) and, to a lesser degree, triiodothyronine (T3). The major portion of T3, however, is produced in peripheral organs, e.g. liver , adipose tissue , glia and skeletal muscle by deiodination from circulating T4.