Effect of Light and of Darkness on Thyroid Gland

Abstract

That darkness or the absence of ultraviolet radiation, particularly of the antirachitic portion, leads to an hyperplasia of the thyroid gland has been suggested by some investigators 1 and denied by others. 2 A critical analysis of these results, however, reveals many discrepancies and contradictions.

The present study extended from November, 1932, through July, 1933, arid involved the use of 140 albino rats. Of this number, 40 were offspring of stock animals bred and kept in the rat room illuminated by diffuse daylight filtered through window glass. The remainder were offspring of rats kept and bred in a dark room, the only radiation coming from a ruby lamp used for a brief period daily while feeding, cleaning, or weighing. The animals were grouped at weaning (21–28 days), weighed weekly, and kept under the various experimental conditions until they were from 12 to 16 weeks old. At the end of each 4-week period, representative animals from each group were sacrificed and the thyroids removed for histological examination. The experiments were divided into 2 series: (1) in which the rats were fed a balanced stock ration, Bal-Ra, rich in iodine; (2) in which the rats were fed bread and fresh milk, a diet relatively poor in iodine. In the first series, one-half of the animals born in light were allowed to remain in the “light room” and groups exposed for 15 minutes every other day at a distance of 1 meter from a flaming carbon arc, burning “Sunshine” carbons (total energy value of each exposure = 6.51 gm. cal. per sq. cm.; ultraviolet = 0.273 gm. cal. per sq. cm.) and to sunlight from 10 A. M. to 2 P. M. daily. The remaining 20 rats were placed in the dark room and kept there except when they were being irradiated.