Circadian rhythms in the concentration of plasma thyroxine in vitamin A-deprived calves

Over a period of approximately 11 months, a total of 57 24 hour experiments was performed on 5 calves. The animals were fed a vitamin A-free diet and showed the common signs of vitamin A deficiency. During each experiment blood samples were taken hourly and the plasma iodoprotein content of each sample was determined. Each set of 24 values was subjected to Fourier analysis in an attempt to elucidate 24 hour, sinusoidal rhythms in iodoprotein concentration. A rhythm was deemed to be present where the chance of random sampling producing the observed pattern of concentration was less than or equal to 0.05. 25 such rhythms were detected. As the probability of random sampling giving rise to 25 rhythms in 57 tests is extremely small, circadian rhythms are assumed to exist in vitamin A-deficient, as in normal, calves. Rhythms, the characteristics of which are described here, were observed under natural conditions of day and night, and with controlled light and darkness. It is suggested that the occurrence of the rhythms, in relation to the environmental patterns of light and darkness, is better related to other aspects of brain activity than to those concerned with vision.