CIRCADIAN AND ULTRADIAN RHYTHMS OF PLASMA THYROID-HORMONE CONCENTRATIONS IN LACTATING DAIRY-COWS

Circadian and ultradian rhythms of thyroxine and triiodothyronine were analyzed by radioimmunoassay on integrated 15-min blood samples colle cted for 48 h from six lactating dairy cows. Body temperatures were re corded every 1.4 min using radiotelemetry. The cows were housed in an environmental chamber at 19.0 +/- 0.5 degrees C (lights on between 070 0 and 2300 h), fed daily at 0900 h, and milked at 0800 and 2000 h. Mea n concentrations of plasma triiodothyronine and thyroxine exhibited si milar circadian rhythms with minima (0500-1300 h) and maxima (1700-020 0 h) separated by 12 h. Triiodothyronine peaked 2 h before thyroxine. The thyroxine circadian rhythm preceded a circadian body temperature r hythm by 2 h. Superimposed on the circadian rhythm was an ultradian rh ythm with a 90-min period that was 15-20% of the mean thyroid hormone concentrations. Peak thyroxine and triiodothyronine concentrations wer e 50 +/- 2 and 1.58 +/- 0.17 ng/ml, and minimal concentrations were 42 +/- 2 and 0.94 +/- 0.17 ng/ml, respectively. Our data indicate that p eripheral triiodothyronine is regulated independently of peripheral th yroxine concentration.