Effect of goitrogen administration on the circadian rhythm of serum thyroid stimulating hormone in the rat

Adult male rats were fed a goitrogen, aminotriazole, for 74 days at a dose known to suppress thyroid function completely. At the end of this period, these animals along with matched controls were killed in groups of seven at 3 hourly intervals throughout a 24 hour period, and serum TSH, T₃, T₄ and albumin assayed. No significant circadian rhythms to T₃,T₄ or albumin were found in either, but a highly significant rhythm of TSH was demonstrated both in controls and goitrogen treated groups, with a diminished relative amplitude in the latter. The results indicate that a significant diurnal rhythm of serum TSH persists in the rat despite long term blockade of thyroid hormone synthesis and that the existence of this rhythm is therefore independent of the presence of circulating T₃ or T₄.