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The Effect of Thyroidectomy and Thyroid Feeding on the Milk Secretion and Milk Fat Production of Cows

Department of Biochemistry, University of Toronto, and Department of Animal Husbandry, Ontario Agricultural College, Guelph

The removal of desiccated thyroid glands from the diet of thyroidectomized cows causes a diminution in the amount of milk and milk fat produced. Thyroidectomy itself causes a marked lowering of milk fat secretion. The diminution in milk secretion following removal of the thyroid gland cannot be readily distinguished from that accompanying control operations.

The addition of small amounts of thyroid to the diet of either thyroidectomized or normal cows, when the curve of lactation is falling, causes a rapid rise in the milk and milk fat production to a higher level after which the gradual fall, which is normal during this period in the lactation cycle, continues. During the period of rising milk secretion, which persists some 4 to 6 weeks after parturition, thyroid feeding had no apparent effect on the amount of milk secreted. Slight differences in the amount of milk fat secreted at this time may be related to the amount of thyroid fed. Feeding excessive quantities of thyroid to a thyroidectomized animal caused a diminution in milk secretion and fat secretion. From reduced milk fat percentages at this time it is apparent that the reduction in fat secretion was greater than that in milk secretion.

Lactation was not reduced to zero when thyroid was removed from the diet of the animals in the experiment. The effect of deprivation of thyroid on the total quantity of milk and milk fat produced seemed to depend on the period in the lactation cycle when the thyroid was removed.

From the results of these experiments it appears that the effect produced by the thyroid glands on milk and milk fat secretion is secondary to those factors controlling the lactation cycle. The experimental results suggest a relationship between total metabolic rate and the secretion of milk and milk fat. Fat production in these experiments appears to be more affected by the presence or absence of thyroid in the diet than is milk secretion. These rapid changes in fat secretion are thus reflected in changes in the per cent of fat in the milk. Hence it is possible that the characteristic fluctuations in fat production, and especially fat percentage, in the records of normal cows, may be due to slight changes in the total metabolic rate of the animals.