The concentration of free thyroxine (FT₄) in serum, measured by a direct method, was 1.77 ± 0.25 mμg/100 ml (mean ± SD) in normal euthyroid adults, 8.47 ± 2.87 in hyperthyroid patients, and 0.54 ± 0.29 in hypothyroid patients. Pregnant euthyroid patients and normal women taking estrogens had increased values for proteinbound iodine (PBI) in serum, while values for the resin uptake of liothyronine I 131 (RT₃) were decreased; but free thyroxine values were normal or slightly decreased. Euthyroid patients with various chronic diseases had decreased PBI values in association with increased RT₃values, but free thyroxine values were normal. Acutely or severely ill euthyroid patients had increased free thyroxine values that often exceeded the normal range. The correlation between values for free thyroxine and the PBI•RT₃product was good in all groups except the sick euthyroid patients. The PBI•RT₃product closely reflected the state of thyroid function in all of the groups of patients.
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Bernhard G. Anderson (1968) studied this question.