Diurnal rhythm in total serum thyroxine levels.

Abstract

The diurnal variation of serum thyroxine, plasma proteins, and hematocrit were studied in six patients (euthyroid, hypothyroid, and thyrotoxic) and in two obese subjects during total fast. Variations of 7% to 34% were observed in serum total protein, hematocrit, protein-bound iodine, total thyroxine, thyroxine binding globulin, and prealbumin binding capacity, and thyroxine dialyzable fraction, but not in the concentration of serum free thyroxine. Similar and parallel diurnal variation was observed during studies of the disappearance of 131I-labeled thyroxine and 125I-labeled albumin. The maximal daily concentration occurred between 10:00 a.m. and 2:00 p.m., and the minimum occurred around 2:00 a.m. These diurnal variations were not related to food intake or adrenocortical function but were influenced by posture. A reversed cyclic change could be produced by reversing the normal sleep-waking pattern. Because of reciprocal changes in the concentration of total serum thyroxine and thyroxine dialyzable fraction, the serum-free thyroxine concentration remained constant. These rhythmic changes are apparently caused by movement of fluid into and out of the vascular compartment. This factor may be an important cause of scatter in the observations recorded during studies of slowly diffusible plasma constituents if repeated blood sampling is not performed under the same conditions. The diurnal variation of serum thyroxine, plasma proteins, and hematocrit were studied in six patients (euthyroid, hypothyroid, and thyrotoxic) and in two obese subjects during total fast. Variations of 7% to 34% were observed in serum total protein, hematocrit, protein-bound iodine, total thyroxine, thyroxine binding globulin, and prealbumin binding capacity, and thyroxine dialyzable fraction, but not in the concentration of serum free thyroxine. Similar and parallel diurnal variation was observed during studies of the disappearance of 131I-labeled thyroxine and 125I-labeled albumin. The maximal daily concentration occurred between 10:00 a.m. and 2:00 p.m., and the minimum occurred around 2:00 a.m. These diurnal variations were not related to food intake or adrenocortical function but were influenced by posture. A reversed cyclic change could be produced by reversing the normal sleep-waking pattern. Because of reciprocal changes in the concentration of total serum thyroxine and thyroxine dialyzable fraction, the serum-free thyroxine concentration remained constant. These rhythmic changes are apparently caused by movement of fluid into and out of the vascular compartment. This factor may be an important cause of scatter in the observations recorded during studies of slowly diffusible plasma constituents if repeated blood sampling is not performed under the same conditions.