Free T4 Index (T7) is a calculated value that estimates the amount of free thyroxine in the blood. This value can help assess thyroid function.

iollo markers that associate with Free T4 Index (T7)


Thyroid hormones stimulate protein catabolism and the release of amino acids like alanine from skeletal muscle. Higher free T4 levels may increase circulating alanine.

Lactic acid

Thyroid hormones increase metabolic rate and glycolysis, which can lead to higher production of lactic acid. Elevated free T4 may increase lactic acid levels.


Phenylalanine is converted to tyrosine by phenylalanine hydroxylase, which is stimulated by thyroid hormones. Higher free T4 may reduce phenylalanine levels by promoting its conversion to tyrosine.


Tyrosine is produced from phenylalanine by the enzyme phenylalanine hydroxylase, which requires tetrahydrobiopterin (BH4) as a cofactor. Thyroid hormones upregulate BH4 synthesis, so higher free T4 levels may increase tyrosine.