Free T3:Free T4 Ratio is Greater Than the Sum of its Parts

Assessing the ratio between the thyroid hormones free T3 and free T4 provides information about thyroid metabolism and the nature of thyroid dysfunction if present. The ratio also provides information about non-thyroidal issues, including the risk of metabolic syndrome and cardiovascular disease.

A reduction in the production of T3, in general, can occur with insufficiency of selenium, a nutrient required for the conversion, or dysfunction of deiodinases, the enzymes that participate in the conversion. A reduction in the deiodination of T4 to T3 will decrease the ratio and may be associated with a hypothalamic-pituitary-thyroid axis adaptation to stress. It should be noted that cortisol decreases the conversion of T4 to T3, thereby lowering the FT3:FT4 ratio.

A low FT3-to-FT4 ratio can be associated with decreased conversion of T4 to T3, selenium insufficiency, low T3 syndrome, adaptation to stress, low muscle mass, cardiovascular disease, and an increased risk of major cardiovascular and cerebrovascular events.

An elevated ratio can be associated with hyperthyroidism, fatty liver, insulin resistance, metabolic syndrome, arterial stiffness, and cardiovascular disease.

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