Oral feeding of White Leghorn hens on a thyroid preparation generally had only a very small toxic effect when given in one dose. A higher death rate occurred when given in small doses on consecutive days.
The hens fell into a moult, varying from slight to complete, 7 or 8 days after thyroid feeding. Severity depended on dose, season and feeding. The sensitivity to moult induction increased gradually from January to May, reaching maximum in summer. During natural moult in September hens were almost insensitive to thyroid preparation. This resistance decreased gradually until February-March.
After giving thyroid preparation, 1-3 eggs were produced, then followed a temporary interruption in egg production, varying in length with the individual, but usually in almost direct proportion to the quantity given. The maximum interruption lasted about 1.5 months despite larger doses. After moulting this maximum diminished to about 3 weeks. After an induced moult the birds rarely moulted again in autumn; yet egg production ceased as usual. Thus advanced moult cannot increase winter production. After induced interruption, normal egg production was resumed.
The influence of thyroid feeding on moult periodicity was probably controlled by another mechanism than that on egg production. A hypothesis was drawn up to explain these mechanisms.