Circadian rhythm of mitotic activity in normal and hyperplastic rat thyroid

This study was designed to provide a reliable quantitative assessment of the circadian rhythm of mitotic activity in the follicular cell population of the rat thyroid, and the effect on this rhythm of prolonged stimulation by a raised level of circulating TSH, induced by goitrogen administration. Mitotic activity in groups of control and goitrogen-treated rats was assessed by a stathmokinetic technique during four 4-h periods spaced equally through one 24-h cycle. Particular attention was paid to the method of sampling to eliminate systematic and minimise random errors, and to the assessment of rhythmicity which was carried out by an appropriate statistical method. A highly significant circadian rhythm was found in control animals with a daytime peak (12.00 to 16.00 h). Goitrogen treatment led to a 5- to 6-fold increase in the mean but a loss of detectable rhythmicity. The results show that the presence and timing of this circadian rhythm must be taken into account in future studied of thyroid growth, and they throw some light on the possible mechanisms of its control. Comparison of the rhythm with that of serum TSH reported previously raises the possibility of a dominant control by this hormone even in euthyroid animals and suggests that it may act on cells in the G2 and/or G1/G0 phases of the cell cycle.