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)
Interpretation:
• 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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Specialty: Endocrinology, Clinical Laboratory
ICD-10: ,
