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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.
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 ]
Regular blood tests are recommended to verify the appropriateness of the dose. [1] They contain a mixture of thyroxine (T4) and triiodothyronine (T3). [1] Desiccated thyroid has been used since the late 1800s. [3] It is usually made from pigs, sheep, or cows. [4] It is available as a generic medication. [1]
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 ...
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]
Recent research suggested the existence of an additional feedforward motif linking TSH release to deiodinase activity in humans. [6] [7] [8] The existence of this TSH-T3 shunt could explain why deiodinase activity is higher in hypothyroid patients and why a minor fraction of affected individuals may benefit from substitution therapy with T3. [9]
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.
TSH is the preferred initial test of thyroid function as it has a higher sensitivity to changes in thyroid status than free T 4. [60] 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, [61] with "a variation in TSH by a mean of between 0.95 mIU/mL to 2.0 mIU/mL ...
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