The Effects of Continuous Light or Darkness on Thyroid Function in Mice12

SEASONAL fluctuations in thyroid function have usually been attributed to temperature changes because of the demonstrated action of heat and cold on thyroid activity. Although the effects of seasonal changes in light-increment on thyroid secretion have not yet been investigated, it seems possible that this too may be a factor. It is also possible that some -of the reported effects of light on gonadal activity may have been due, in part at least, to light-induced changes in thyroid secretion. That alterations in thyroid function can induce profound changes in gonadal activity has been emphasized in recent reports by Meites et al. (1949) and Maqsood et al. (1950). The available data on the effects of light on thyroid function are not in agreement. Bergfeld (1930) stated that absence of ultraviolet light caused thyroid hypertrophy in rats irrespective of environmental temperature. In twelve female rats which were kept in continuous light for one month,