The 24-Hour Plasma Thyroxin Profile in Normal Man

Plasma T4 concentrations were measured by radioimmunoassay at 20-min intervals over a 24-hr period in 4 euthyroid men and over a 48-hr period in 1 euthyroid man. Application of the frequent sampling technique to the study of circulating T4 has demonstrated the existence of acute, short duration plasma T4 fluctuations in all 5 subjects. The mean T4 concentrations were slightly higher during wakefulness than during sleep in all subjects with the exception of the second 24-hr period in the 48-hr study. The magnitude of the fluctuations ranged from changes minimally detectable by the assay to 1.5–2.0 μg/dl.