Absence of Diurnal Variation of Serum TSH

Frequent sampling of blood throughout the day and night has been carried out in normal subjects and in patients with mild or severe primary myxedema. Measurements of TSH (by radioimmunoassay) and plasma cortisol have been made on each blood sample. Apart from what appear to be very occasional random, but significant variations in serum TSH concentration, no convincing pattern of circadian or other mode of variation of TSH secretion could be identified. On the contrary, our observations indicate that in most subjects, normal or hypothyroid, serum TSH levels remain remarkably constant, being unaffected by activity, sleep or eating.