The Role of Thyroidal Materials and of Synthetic Goitrogens in Animal Production and an Appraisal of their Practical Use (1949)

The literature on the physiologic effects of thiouracil and thyroidal substances has been reviewed, with particular attention to possible applications of these compounds in the principal classes of livestock, namely, dairy cattle, beef cattle, sheep, swine, and poultry. By virtue of its thyroid-inhibiting property thiouracil will cause a depression of milk and egg production and when given to very young animals will cause growth stasis and varying degrees of cretinism. When given only during the last three to four weeks of the fattening period in chickens and turkeys, however, and the last four to six weeks in swine, this drug has caused significant improvement in the economy of gains in most cases with little depression in the rate of gain. Increased fat deposition has usually been observed in poultry, but not in swine. Results with thiouracil in beef cattle and sheep have been inconsistent, presumably because the optimal conditions for administration of the drug have not been established. The use of thyroidal substances for the stimulation of milk production has been investigated quite extensively. Within a limited range the percentage increase in milk yield is roughly proportional to dosage. Fat percentage also increases, the increase usually appearing at lower dosage levels than the rise in milk yield. With the exception of a small and variable decrease in milk nitrogen no other important alterations in milk composition have been reported. Thyroid hormone is not transmitted to the milk. It is concluded that milk from cows fed iodinated protein is safe and nutritionally adequate. It is well established that thyroidal stimulation of dairy cows must be accompanied by an increase in nutrient intake if losses in body weight are to be avoided. If sufficient extra nutrients are supplied to support both the increased metabolism and the increase in production, normal body weights are maintained. It is extremely important to keep the dosage within the “physiological tolerance range” of the cow, particularly in long time feeding programs, in order to avoid overdosage. On the basis of present information it appears that thyroidally active iodinated protein could be used profitably during periods of milk scarcity to stimulate production for periods of about two months. Under good conditions of feeding and management and proper regulation of dosage it could be used in commercial herds for more extended periods. Until more information is available on the long term effects, the use of iodinated protein in purebred breeding herds would appear questionable. Iodinated protein is poorly tolerated during hot weather. However, further research at low levels of dosage would be of interest. Iodinated protein has proved beneficial in improving libido and fertility in sluggish and inactive bulls, and may provide a means of extending the period of usefulness of some herd sires. It has also been found to overcome “summer sterility” in rams although no practical experiments on the use of this method for lengthening the breeding season of sheep have been reported. Conclusive information on the effects of iodinated protein on lactation in sows has not thus far been reported. When fed to young growing pigs, within a critical dosage range, iodinated protein has caused some increase in the growth rate together with a slight improvement in economy of gains. The pigs appear thicker bodied, more mature and more uniform in conformation than control animals. Results on carcasses range from no effect to some increase in weight of loin and bacon cuts. In chicks iodinated protein consistently causes a pronounced acceleration of feather growth. Effects on body growth are variable, depending upon the dosage level given, the breed and strain of chicks and probably other factors. Small amounts of iodinated protein given continuously in the ration have been reported to prevent partially the summer decline in egg production and also to retard the yearly rate of decrease in egg production with advancing age. Since agreement among several laboratories is poor, however, more research will be required to assess the possible role of thyroidal stimulation in egg production. Iodinated protein has also been reported to improve egg shell quality, but again there is some variability between laboratories, indicating the need for further research. Because of the newness of this line of investigation any conclusions that can be drawn at this time as to the potential usefulness of alterations of thyroid function in the various fields of animal production are definitely limited in their scope. As in all new fields of investigation the solution of one problem invariably calls attention to several others that are awaiting solution. The introduction of goitrogens and thyroactive iodinated proteins, which permit control of the thyroid function at will, not only gives promise of some applications of economic importance, but also provides a research tool that should help to advance a step farther the frontiers of our knowledge of the physiologic processes upon which livestock production is based.