The ratio of thyroxine-to-thyroxine binding globulin approximates the concentration of free thyroxine.


thyroxine-to-thyroxine binding globulin ratio =

= (thyroxine in µg/dL) / (thyroxine binding globulin in mg/dL)



• normal ratio: 3-5

• The ratio is increased in hyperthyroidism, congenital thyroid binding globulin excess, and familial hyperthyroxinemia.

• The ratio is decreased in hypothyroidism, congenital thyroid binding globulin deficiency, and during the third trimester of pregnancy.

• The ratio tends to be unchanged with acquired changes in thyroid binding globulin due to parallel changes in thyroxine.


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