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 collected for 48 h from six lactating dairy cows. Body temperatures were recorded every 1.4 min using radiotelemetry. The cows were housed in an environmental chamber at 19.0 +/- 0.5 degrees C (lights on between 0700 and 2300 h), fed daily at 0900 h, and milked at 0800 and 2000 h. Mean concentrations of plasma triiodothyronine and thyroxine exhibited similar circadian rhythms with minima (0500-1300 h) and maxima (1700-0200 h) separated by 12 h. Triiodothyronine peaked 2 h before thyroxine. The thyroxine circadian rhythm preceded a circadian body temperature rhythm by 2 h. Superimposed on the circadian rhythm was an ultradian rhythm with a 90-min period that was 15-20% of the mean thyroid hormone concentrations. Peak thyroxine and triiodothyronine concentrations were 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 peripheral triiodothyronine is regulated independently of peripheral thyroxine concentration.