Nyctohemeral and Sex-Related Variations in Plasma Thyrotropin, Thyroxine, and Triiodothyronine

Abstract

TSH, T4, and T3 were measured by radioimmunoassay in plasma samples obtained from 77 young adult male and 114 female rats fed a Purina high-iodine diet and maintained in an isolated room, 2-4/cage, at 24 +/- 1 C with light from 0600-1800 h. In one experiment, 7 male and 7 female rats were decapitated every 3 h for 30 consecutive h and trunk blood was collected. There was a clear nyctohemeral rhythm of plasma TSH in both sexes characterized by a zenith at 1200 h and a nadir between 1800 and 2100 h. The plasma TSH cycle was approximately 180 degrees out of phase and negatively correlated (P less than .05) with that of plasma corticosterone (B) in both sexes. Although glucocorticoids have been reported to suppress TSH secretion, there was no causal relationship between plasma B and TSH in our experiments since the TSH cycles were normal in chronically adrenalectomized rats. Normal TSH cyclicity was not observed in severely iodine-deficient rats with extremely high plasma TSH levels although the nyctohemeral B rhythm was normal. Plasma TSH was approximately twice as high in males as in females (overall mean +/- SE: M = 149 +/- 11, F = 81 +/- 7 muU/ml, p less than 0.001). There was no significant difference (P greater than 0.05) in plasma TSH at different stages of the estrous cycle. Plasma T4 was slightly, but significantly, higher in males than females (overall mean +/- SE: M = 6.4 +/- 0.1, F = 6.0 +/- 0.1 mug/100 ml; P less than 0.001), while T3 was higher in females than in males (overall mean +/- SE: M = 69.5 +/- 1.7, F = 80.3 +/- 2.1 ng/100 ml; P less than 0.001). No significant nyctohemeral rhythm was observed in plasma T4 or T3 in either sex. These observations indicate that: 1) There is a nyctohemeral rhythm of plasma TSH which is independent of plasma B fluctuations and not associated with proportional changes in plasma thyroid hormones. 2) A sustained high rate of TSH secretion abolishes the normal nyctohemeral plasma TSH rhythm. 3) There are significant differences in plasma concentrations of TSH, T4, and T3 between male and female rats.

The importance of sodium/iodide symporter (NIS) for thyroid cancer management

“…Some previous reports indicate that estradiol might regulate thyroid function through a direct action on thyrocytes (34), apart from a possible effect on TSH secretion (35)(36)(37)(38). The administration of estradiol to ovariectomized and hypophysectomized rats leads to a higher thyroid radioiodide uptake (32,39), which supports the hypothesis of a direct effect of estrogen on the thyroid. More recently, the presence of estrogen receptors has been demonstrated in both human and rat thyroid glands (40,41).…”

“…The prevalence of thyroid diseases is higher in women than in men, and in the postmenopausal period a further increase is detected (32,33). Some previous reports indicate that estradiol might regulate thyroid function through a direct action on thyrocytes (34), apart from a possible effect on TSH secretion (35)(36)(37)(38).…”

Morphological changes on the free surface of thyroid cells at the hormone secretion by scanning electron microscopy.

“…The highest blood level of TSH was found at noon and the lower level was kept between 6 p.m. and 9 p.m. in rats controlled in 12-hr day-night cycle (the dark period was 6 p.m. to 6 a.m.) (Fukuda et al 1975). On the other hand, Pollardo et al (1976) reported that the significantly high level of plasma TSH was observed between 2 p.m. and 9 p.m. and the rate of secretion of thyroid hormone was greater in the dark period in rats kept on almost the same condition as above.…”

Effect of acute exposure to cold on the levels of corticosterone and pituitary hormones in plasma collected from free conscious cannulated rats.

“…A circadian periodicity in circulating corticosterone, ACTH and thyrotropin levels is also reported (Guillemin et al, 1959;Cheifetz et al, 1968 ;Hiroshige et al, 1969;Fukuda et al, 1975). Moreover, recent evidence has indicated that rat pituitary hormones such as LH (Blake, 1975) and growth hormone (GH) (Martin et al, 1974;Tannenbaum et al, 1976) are secreted in pulsatile fashion.…”