Free thyroxine in serum in relation to thyroid function

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Abstract
The concentration of free T4 (FT4) in serum, measured by a direct method, was 1.77 [plus or minus] 0.25 m[mu]g/100 ml (mean [plus or minus] SD) in normal euthyroid adults, 8.47 [plus or minus] 2.87 in hyperthyroid patients, and 0.54 [plus or minus] 0.29 in hypo-thyroid patients. Pregnant euthryoid patients and normal women taking estrogens had increased values for protein-bound iodine (PBI) in serum, while values for the resin uptake of liothyronine I 131 (RT3) were decreased; but FT4 values were normal or slightly decreased. Euthyroid patients with various chronic diseases had decreased PBI values in association with increased RT3 values, but FT4 values were normal. Acutely or severely ill euthyroid patients had increased FT4 values that often exceeded the normal range. The correlation between values for FT4 and the PBI[center dot]RT3 product was good in all groups except the sick euthyroid patients. The PBI[center dot]RT3 product closely reflected the state of thyroid function in all of the groups of patients.