CHANGES IN THYROID HORMONES DURING THE EGG LAYING CYCLE IN FAYOUMI HENS

T, and The aim of the present study was to determine the levels of T TSH in the serum of laying and non laying Fayoumi chickens. The biological
and T, was also measured in these chicken to half life of exogenous T elucidate the relationship between the thyroid hormones and egg production.
The results obtained revealed that: The level of serum T during the egg laying cycle varied significantly. At the time of laying, serum T was relatively low and still low at 12 h. Then it reached its maximal level at 20 h after egg laying. The levels of T, in serum did not show any significant variation during all stages of egg laying cycle. Significantly high level of serum Ty was observed in non-laying hens. The lowest level of TSH was observed at egg laying, while the maximal level was observed a time around 12 h after egg laying. In non-laying hens serum TSH was low. The biological half-life of T, was long in hens 20 h from egg laying, while it was shorter at egg laying. In non-laying hens trio of T, was very long. The rate of disappearance of T, from circulation of laying hens was more rapid at 12 h after egg laying. It is not simply a matter of increased or decreased secretion of T, and Ty during the egg laying cycle, but extends to selective absorption of in hibition of absorption of T and T, by alimintary tract. It includes also variations in utilization of T and T at a certain time in the egg laying cycle which is different from non laying hens.