THYROID DISEASES-II -FTI TESTS

FREE THYROID INDEX TEST-FTI

This is an estimation of free thyroid hormone most importantly T-4 which is the major contributor of the thyroid secretion but minor contributor of the effects of thyroid.

Usually the secretion of T-4 is more in hyperthyroidism and less in hypothyroidism as there may not be much change in the secretions of T-3.

Hence T-4 estimation which is freely circulating in the blood is very important index of assessing the patient's condition.Even though the effects of thyroid is mostly contributed by T-3 it is not important to asses it as a factor because most of the T-3 conversions from T-4 happens only in circulation and not within the thyroid gland

Also this not a separate test rather an estimation of T-4 level which is freely circulating in the blood by mathematical relationship between Resin Triiodothyronine Uptake(RT3U) and serum T-4 levels as follows

FTI = TT-4 x RT-3U/mean serum RT-3U

TT-4 = Total serum T-4;

FTI is higher in hyperthyroidism in which TBG the thyroxin binding globulin is less and vice versa
Alternatively thyroid functions can be assessed by TSH test.TSH the thyroid stimulating hormone is secreted by the pituitary gland according to the thyroid functions.
If pituitary secretes high TSH secretions which elevates the serum TSH level above the normal that means the thyroid gland is underacting which is known as Hypothyroidism.
But if the pituitary secretions of TSH is less which is reflected by the depression of serum TSH level then it means the thyroid gland is over acting which is known as Hyperthyroidism
The normal range of serum TSH level is 3.9 to 4.6 milliunits/Liter.(American Thyroid Association)