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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.
Affected patients may have normal, low, or slightly elevated TSH depending on the spectrum and phase of illness. Total T4 and T3 levels may be altered by binding protein abnormalities, and medications. Reverse T3 levels are generally increased, while FT3 is decreased. FT4 levels may have a transient increase, before becoming subnormal during ...
In sub-clinical hyperthyroidism, serum TSH is abnormally low, but T4- and T3-levels fall within laboratory reference ranges. [47] It primarily affects the skeleton and the cardiovascular system (abnormalities in other systems have also been reported), in a similar but less severe and less frequent way than overt hyperthyroidism does.
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]
Blood free thyroxine and TSH levels are monitored to help determine whether the dose is adequate. This is done 4–8 weeks after the start of treatment or a change in levothyroxine dose. Once the adequate replacement dose has been established, the tests can be repeated after 6 and then 12 months, unless there is a change in symptoms. [ 8 ]
Biotin can cause this test to read "falsely low". [21] Time of day can affect the results of this test; TSH peaks early in the morning and slumps in the late afternoon to early evening, [65] with "a variation in TSH by a mean of between 0.95 mIU/mL to 2.0 mIU/mL". [66] Hypothyroidism is diagnosed more often in samples taken soon after waking. [67]
In such a case a TRH stimulation test, in which TRH is given and TSH levels are measured at 30 and 60-minutes after, may be conducted. [84] T 3 and T 4 can be measured directly. 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 ...
The test is used in the differential diagnosis of secondary and tertiary hypothyroidism. First, blood is drawn and a baseline TSH level is measured. Then, TRH is administered via a vein. After 30 minutes blood is drawn again and the levels of TSH are measured and compared to the baseline. Some authors recommend additional blood sampling at 15 ...