It is well known that there are wide species, sex and age differences in thyroid activity; that marked alteration in thryoid function can be brought about by environmental changes such as temperature; and that alteration of nutritional or endocrine status may affect the activity of the thyroid gland. Less well known are the influences of genetic constitution on thyroid function. Such differences in genetic makeup may be referred to as breed differences within a species, e.g., differences between the White Leghorn, Rhode Island Red, etc. Strain differences, in the case of chickens, generally refer to chickens which have been selected over a period of time for a specific purpose such as rapid-growth, slow-growth, rapid-feathering, etc. Line differences, as in mice, generally refer to lines of mice which have been selected over a period of time for some particular character such as growth rate, mature body size, etc.