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) muU/ml compared with a mean of 1.3 +/- 0.9 muU/ml for the entire baseline records of the 8 subjects. The effect of iv infusion of 32-1000 mug of somatostatin (SRIF) for 1 1/2-3 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 is less than 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 mug bolus infusion of T4 raised the basal T4 level by only 1-2 mug/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.

Effect of a single dose of glucocorticoid on the diurnal variations of TSH, thyroxine, 3,5,3'-triiodothyronine, 3,3',5'-triiodothyronine and cortisol in normal men.

“…The lack of diurnal variation in plasma T4and T3levels agrees with the results of previous reports (Azukizawa et al, 1976;O'Connor et al, 1974). Our results by the6hr interval sampling for2days do not suggest the possibility of diurnal variation of plasma reverse T3concentration.…”

“…Computational analysis of the data failed to reveal any correlations between the diurnal rhythm of plasma TSH levels and the fluctuations of thyroxine (T4) or3, 5, 3'-triiodothyronine (T3) in plasma (Azukizawa et al, 1976). It has been suggested that the circadian rhythm of TSH secretion is the result of the inhibitory action of plasma cortisol because there is an inverse relationship between their circadian patterns (Nicoloff et al, 1970), and because of the suppressive effect of dexamethasone on TSH secretion (Wilber and Utiger, 1969).…”

Fluctuation of the plasma TRH level in normal subjects in a 4-hour observation period.

“…We found that there was no correlation between the total thyroid hormone levels and TSH levels in samples taken at 20-min intervals (Azukizawa et al, 1976). Recently Weeke and Gundersen observed that the diurnal rhythm of serum free triiodothyronine and serum TSH levels were synchronous with low levels in the daytime and high levels at night (Weeke and Gundersen, 1978).…”

“…Since the development of a highly sensitive radioimmunoassay for hTSH has made it possible to measure the basal level of serum TSH, a diurnal variation in serum TSH has been demonstrated in normal subjects (Nicoloff et al, 1970 ;Patel et al, 1972 ;Weeke, 1973 ;Azukizawa et al, 1976). However, the role and mechanism of the diurnal change in the serum TSH concentration are still unknown.…”

Effect of .GAMMA.-oryzanol on serum TSH concentrations in primary hypothyroidism.

“…After the baseline blood samples were obtained, 7hypothyroid patients were orally given300mg of r-oryzanol (Ohtsuka Pharmaceutical Co., Japan) and blood samples were subsequently drawn at1, 2, 4and6hr. Three days later, blood was taken from these patients at Diurnal TSH variations in man was recently shown by some investigators (Weeke and Laurberg, 1976;Azukizawa et al, 1976;Weeke and Gundersen, 1978) employing single antibody wick-chromatographic radioimmunoassays (Weeke and rskow1973) or high-sensitivity radioimmunoassay technique for TSH (Pekary et al, 1975 ic action of r-oryzanol (Itaya et al, 1976).…”