618 



PHYSIOLOGY OF GONADS 





10 



50 



50 



Dau5 



Fig. 10.16. Effect of L-thyroxine given in the feed on the milk yield of groups of cows 

 (the indicated dose levels were fed daily). (From G. L. Bailey, S. Bartlett and S. J. Folley, 

 Nature, London, 163, 800. 1949.) 



topoietic when ]ed to lactating cows in daily 

 doses of about 100 mg. (Fig. 10.16). It had, 

 moreover, none of the drawbacks of the 

 iodinated proteins, its purity could be 

 checked chemically, it was odorless and 

 tasteless. AVith the introduction of syn- 

 thetic thyroxine, iodinated proteins have 

 become obsolete as galactopoietic agents. 



The more recently isolated 3:5:3-tri- 

 iodo-L-thyronine, reported to be 5 to 7 

 times more active than thyroxine in various 

 biologic tests in small animals and also in 

 man, has little or no effect on the milk yield 

 when fed to cows, but is somewhat more 

 active than thyroxine in promoting galacto- 

 poiesis when administered subcutaneously, 

 which suggests that the material is inacti- 

 vated in the gut, probably in the rumen 

 f Bartlett, Burt, Folley and Rowland, 1954). 



The extensive experiments on galacto- 

 poiesis in dairy cattle with thyroxine and 

 thyroid-active substances have made it 

 possible to reach reasonably firm conclu- 

 sions as to the practical value of the pro- 

 cedure. There is great variability in the 

 response to treatment; in general a better 

 response is ol)taincd during the decline of 

 lactation than at the peak and end of lac- 

 tation. The use of thyroid-active substances 



in animals undergoing their first, second, 

 or third lactation is of doubtful benefit be- 

 cause the boost in yield is largely cancelled 

 out by a shortening of the lactation pe- 

 riod. Short-term administration at suitable 

 times can result in considerable galactopoie- 

 sis, but this is frequently followed by marked 

 falls in yield when the administration of 

 thyroid-active material ends. The admin- 

 istration of thyroid-active materials to 

 dairy cows, if carried out with due care, 

 has no ill effects on the health and repro- 

 ductive abilities of the cows (see Leech 

 and Bailey, 1953) , but because of the rather 

 small net gain in yield (about 3 per cent) 

 the practical application of the procedure 

 seems to be limited. 



The mode of action of thyroxine and 

 thyroid-active substances on milk secretion 

 is uncertain. It is tmlikely that it is a 

 specific effect on the alveolar cells; rather 

 is it probably related to tlie effects of 

 the thyroid hormone on the general meta- 

 bolic rate. 



E. PARATHYROm HORMONE 



The early studies on the influence of the 

 parathyroid glands on milk secretion indi- 

 cated, as might be expected from their 



