Free T3 is the form of thyroid hormone that is most bioavailable and ready to do all those things that the thyroid does including regulating metabolism, energy expenditure, weight, and cardiometabolic health.
Evaluating free T3 can be complex because the amount in circulation depends on the conversion of T4 to T3, a micronutrient-dependent process, as well as the amount of T3 that becomes bound to transport proteins, e.g., thyroid binding globulin, albumin, and prealbumin.
A low FT3 is associated with hypothyroidism, non-thyroidal sick syndrome, malnutrition, liver disease, diabetes, inflammation, and toxin exposure. An elevated FT3 is seen with hyperthyroidism and may be associated with insulin resistance, decreased lean body mass, and the use of heparin which can increase free fatty acids and dislodge T3 from its binding proteins.
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