Thyroxine (T4) and Triiodothyronine (T3)

     Hormonal production of the thyroid gland is constituted of thyroxine or T4 (80%) and triiodothyronine or T3 (20%). In the circulation, whole T4 originates from thyroid secretion but most of T3 (80%) is produced outside the thyroid from T4 deiodination. Conversion of T4 to T3 may be influenced by various conditions and circulating T3 is a less reliable reflection of thyroid hormone production than T4. In blood most of T4 and T3 is bound to binding proteins and Only 0.02% of T4 and 0.3% of T3 is free. Because of their higher diagnostic performance, free T4 (FT4) and free T3 (FT3) measurements have superseded total (free + bound) hormone determination.

    Free T3 – measures the free T3 hormone levels. This test can be really useful for finding out what amount of active thyroid hormones are available for the thyroid receptor sites. Free T3 blood test is high in hyperthyroid conditions and low in hypothyroid conditions. May also be high in thyroid toxicosis.

    Free T4 – used to measure the amount of free or active T4 in the blood. High with hyperthyroidism, low with hypothyroidism. The drug Heparin can also cause elevated free T4 as can some acute illness. It’s also high in an overdose of thyroid hormone.

Normal total T4 range

·        - 5-10.9 g/dl for mhttps://go.hotmart.com/V94904233Ken and women

When T4 blood test is high

·        -Too much thyroxine is commonly associated with hyperthyroidism

When T4 blood test is low

·       -  This can be a sign of a poorly functioning thyroid gland or hypothyroidism

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