Plasma Thyrotropin, Thyroxine, and Triiodothyronine Relationships in Man

Abstract

The physiologic relationships of plasma TSH, T₄ and T₃ levels measured every 20 min in seven healthy young men and one healthy young woman have been investigated. A nocturnal TSH surge was observed in all subjects on both nights of the 36–48 h baseline observation period. In males the maximum plasma TSH value occurred at 2300 h. The mean peak TSH level was 2.0 ± 0.3 (SE) μU/ml compared with a mean of 1.3 ± 0.9 μU/ml for the entire baseline records of the 8 subjects. The effect of iv infusion of 32–1000 μg of somatostatin (SRIF) for 1½–3 h was investigated in four of the male subjects during 2 or 4 consecutive nights following the control period. Temporal relationships between the hormonal fluctuations observed throughout the control period and during the nights of SRIF infusion were investigated using time series analysis and Student's t test. Rapid fluctuations of plasma T₄ and T₃ concentration were noted, even when corrected for changes in total protein concentration, with an average coefficient of variation of 10% for T₃ and 12% for T₄. No increment of plasma T₄ or T₃ followed the nocturnal TSH surge nor were the rapid fluctuations of the thyroid hormones altered by the TSH surge. SRIF infusion commencing at 2300 h suppressed the elevated TSH levels (P < 0.01) while similar infusions begun at 2100 h blocked the expected nocturnal TSH rise observed during control periods in male subjects. Plasma T₄ and T₃ levels were not significantly affected by the administration of SRIF. The relationship of the rapid plasma T₄ and T₃ variations to postural changes was investigated in four euthyroid male subjects. Serum levels of TSH, T₄ and T₃ and total protein were determined at 15 min intervals while postural changes were carefully monitored. The ratios of T₄ and T₃ to total protein were relatively stable (3–4% coefficient of variation) when the subjects were kept in a supine and motionless position. A 50 μg bolus infusion of T₄ raised the basal T₄ level by only 1–2 μg/dl. The data suggest that short-term fluctuation of plasma T₄ and T₃ result from changes in protein concentration due to hemodynamic responses to alteration of posture and physical activity and not to pulsatile secretion of T₄ and T₃.