PHYSIOLOGY OF THE THYROID GLAND IN BIRDS: A REVIEW.

Summary. A brief introductory account of the structure of the thyroid gland and the general effects of its hormone are given. There is sufficient experimental evidence to warrant the conclusion that in birds, as in other vertebrates, the level of thyroid function is regulated by a hormone of the anterior lobe of the pituitary gland, the thyrotrophic hormone. Increased thyroid activity during the cooler period of the year is found in a number of species, and this is probably widespread. For all species so far tested, feathers grown during thyroid deficiency show abnormally loose texture and become elongated and narrow. Reverse changes can be produced by thyroid administration to normal birds. The relationship between the thyroid and migratory behaviour, though probable, needs further investigation. Feather pigmentation is in some species unaffected by the thyroid, but is increased in others. There is undoubtedly a relationship between the moult and increased thyroid activity since a phase of high thyroid function preceding the moult is of widespread occurrence. In some gallinaceous birds the moult can be precipitated by thyroid administration, but not in ducks. In some species complete suppression of the moult has been claimed if thyroidectomy is performed sufficiently far ahead of the next moult due. Complete dependence of moulting on a raised output of thyroid hormone has, however, not yet been proved for any species. So far there is experimental evidence, which requires comfirmation, from one species only that return of the gonad to the resting condition following the breeding‐season recrudescence is to a considerable extent dependent on the thyroid. In Domestic Fowls thyroid hormone affects the spur.