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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.
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.
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 ]
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.
Percentiles for Jostel's TSH index (TSHI or JTI) along with reference ranges for thyroid's secretory capacity (SPINA-GT) and univariable reference ranges for thyrotropin (TSH) and free thyroxine (FT4), shown in the two-dimensional phase plane defined by serum concentrations of TSH and FT4.
The therapeutic target range TSH level for patients on treatment ranges between 0.3 and 3.0 μIU/mL. [18] For hypothyroid patients on thyroxine, measurement of TSH alone is generally considered sufficient. An increase in TSH above the normal range indicates under-replacement or poor compliance with therapy.
The first-trimester normal range is below 2.5 mIU/L and the second and third trimesters normal range is below 3.0 mIU/L. [16] [44] Treatment should be guided by total (rather than free) thyroxine or by the free T 4 index.