Daily doses of 4 mg. thyroid iodine per 1000 and 2000 gm. of body weight proved lethal to male and female fowls.
Daily doses of 4 mg. thyroid iodine per 3000 to 5000 gm. body weight caused loss in weight in males and females. The same amount of thyroid iodine to 7000 gm. of body weight caused loss of weight in a cockerel but not in a hen. Smaller doses had no effect on the weight in either sex.
All doses, even 4 mg. thyroid iodine to 10,000 gm. body weight, caused hen feathering in the males.
Depigmentation was quite marked in the case of the heavier doses, but less evident in that of the smaller. In general, depigmentation was most evident in the birds which declined in weight. The minimum daily dose necessary to produce marked depigmentation was 4 mg. thyroid iodine to 5000 gm. of body weight. This explains why several investigators have not obtained the depigmentation described by Giacomini and Zavadovsky.
On doses of 0.8 mg. thyroid iodine per bird or less, Cole and Reid, and Crew observed production of darker feathers. On much larger doses Giacomini, Zavadovsky and the writer observed depigmentation.
It would seem that the smaller doses of thyroid cause increased production of melanin, presumably by the general increase in metabolic processes, but that at a certain stage (which this experiment indicates to be around 4. mg. thyroid iodine per 5000 gm. of body weight) the production of pigment is arrested.
The use of a definite dosage of thyroid iodine based on body weight has led to consistent results and to an explanation of some of the discrepancies which have previously appeared in the literature.