Does Thyroid Affect Milk—Egg Production?
Recognition of the significance of the thyroid gland in animals, the association of iodine with the thyroid, and the availability of an excellent radioisotope of iodine have resulted in increased study of this important gland. Chemical tests had hinted at a link between the thyroid gland and the production of milk and eggs. Using radioactive iodine, scientists learned that thyroid activity increases with the onset of milk and egg formation. In hot weather, when yield of milk and eggs decreases, activity of the thyroid gland diminishes.
It may be that a dairy breeder can soon select calves for potential milk production because of thyroid activity as measured by radioactive iodine. At present he must let the animals grow and produce milk for several years before he chooses those to use in herd improvement. (See [Fig. 4].)
More Tracers in Animal Nutrition Research
Female hormones in microgram amounts[1] accelerate fattening of cattle and sheep. Before this method can be used on animals for human consumption, however, it must be determined that no possible human injury can result from any residue. With chemical tests the measurement of such tiny amounts was impossible. Even with radioactive carbon-14, doses of hormone 1000 times normal dosage were required before the hormones in the flesh were measurable.
Fig. 4—Future high-producing milk cows may be selected as calves, because of the measured activity of their thyroid glands. A minute amount of iodine-131 is fed and within minutes has concentrated in the thyroid. High concentration means high thyroid activity, which in older animals means high milk production.
Recently an isotope of hydrogen (tritium or H³) was linked to hormones, and these were fed in normal amounts to cattle. Tests 90 days afterward showed less than one part per billion in the meat, a big step toward cheaper fattening of meat animals with hormones.
The use of tranquilizers has been proposed for reducing the customary loss of weight in cattle being shipped to market. These chemicals, also, are used in such minute quantities that residues could not be detected except with radioactive tracers. Tritium can now be used by health officials to study the effects of tranquilizers.