RELATIVE GALACTOPOIETIC EFFECTS OF 3:5:3′-TRIIODO-l-THYRONINE AND l-THYROXINE IN LACTATING COWS

Abstract

SUMMARY 1. The relative potencies of l-triiodothyronine (TIT) and l-thyroxine in evoking the various phases of the galactopoietic response (increase in milk yield and in the percentages of milk fat and non-fatty solids, decrease in the milk alkaline phosphatase level) and tachycardia, have been compared in lactating cows. 2. By the oral route, a dose of 64 mg TIT daily was very much less effective than an approximately equimolar amount of l-thyroxine (75 mg daily), as judged by effects on milk yield, fat and phosphatase. Neither substance affected the milk non-fatty solids. Thyroxine caused a progressive increase in heart rate throughout the feeding period, while TIT evoked an immediate tachycardia which at first equalled that eventually produced by thyroxine, but thereafter rapidly declined even while feeding of TIT was continued. 3. By the subcutaneous route, 5 mg TIT once daily were slightly more active than 5 mg l-thyroxine once daily in evoking effects on milk secretion (including, in this case, an increase in milk non-fatty solids content) and appreciably more active in producing tachycardia. When 2·5 mg of these substances were injected twice daily, the results were almost identical so that division of the dose did not favour the action of TIT as compared with l-thyroxine. 4. It is suggested that the relatively low oral potency of TIT as compared with l-thyroxine in cows is due to its greater solubility in the rumen contents, the micro-organisms of which quickly become adapted to bring about its inactivation.