Effect of thyroidectomy on the secretory profiles of growth hormone, thyrotropin and corticosterone in the rat.

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Abstract
Changes in pituitary hormone secretions following thyroidectomy were examined in sequentially collected blood samples every 15 min day and night for 7 h from male rats via cannulae into the right atria. Pulsatile growth hormone secretions were observed every 2.5 or 3.5 h in normal rats, while thyroidectomy 3 wk prior to experiments abolished the growth hormone burst. Corticosterone profiles from normal rats showed also episodic patterns which were more often observed in the night experiment. The episodic secretion of corticosterone seemed to inhibit the subsequent growth hormone burst but not vice versa. The interrelation between integrated concentrations of growth hormone and corticosterone showed a negative correlation although the coefficient was not statistically significant. After thyroidectomy, the basal level of corticosterone decreased and the episodic secretion was blunted. There was no nyctohemeral variation in the plasma thyrotropin level in normal rats but small and slow fluctuations. Thyroidectomy caused about 10-fold elevation in the basal thyrotropin level and, in addition to this, a decrease at 1500 h and an increase during the night, followed thereafter by a decreasing tendency to 0400 h. Power spectral analysis revealed smaller components in growth hormone secretion of normal rats, which had periods of 60, 40 and 36 min, besides the hormone burst. The spectral analysis apparently showed that there were small and frequent components (40 or 36 min period) in corticosterone secretion.

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