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The Free Thyroxine Index (FTI or T7) is obtained by multiplying the total T 4 with T 3 uptake. [2] FTI is considered to be a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding. [2] This test is rarely used now that reliable free thyroxine and free triiodothyronine assays are routinely available.
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.
The characteristic blood test results for this disorder can also be found in other disorders (for example TSH-oma (pituitary adenoma), or other pituitary disorders). The diagnosis may involve identifying a mutation of the thyroid receptor, which is present in approximately 85% of cases. [8]
The results suggest that bone loss could be an effect of levothyroxine use in older adults, ... Hypothyroidism is diagnosed by looking at the free thyroxine (T4) levels in people with elevated TSH ...
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.
In addition to testing the TSH levels, many doctors test for T 3, Free T 3, T 4, and/or Free T 4 for more detailed results. Free T 4 is unbound to any protein in the blood. Adult limits for these hormones are: TSH (units): 0.45 – 4.50 uIU/mL; T 4 Free/Direct (nanograms): 0.82 – 1.77 ng/dl; and T 3 (nanograms): 71 – 180 ng
A related parameter is the free thyroxine index, which is total T 4 multiplied by thyroid hormone uptake, which, in turn, is a measure of the unbound TBG. [75] Additionally, thyroid disorders can be detected prenatally using advanced imaging techniques and testing fetal hormone levels.
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 ]