Thyroid Hypertrophy in the Rat with Reference to the Effect of Light. ∗

Abstract

In describing the thyroid hypertrophy and colloid loss in rats exposed to cold and darkness for 10 to 25 days the writer 1 noted that he had not seen any striking changes due to darkness alone. Bergfeld 2 had previously stated that in 4 to 6 weeks, absence of ultraviolet light caused thyroid hypertrophy irrespective of the environmental temperature and that in his small series no effects attributable to cold were found if the room were illuminated. While students using the rat have not agreed on this effect of darkness, conclusive evidence that in the chick absence of ultraviolet light for 55 to 105 days results in marked thyroid hypertrophy and colloid loss has been provided by Turner and Benedict. 3 The interesting possibility thus suggested that light may play some vital rôle in the formation of the thyroid hormone has led the writer to further examine its possible influence under the conditions of temperature and diet used in his earlier work.

From March 8 to June 13, 1933, (98 days) 10 albino rats of about 90 gm. in weight were kept in a dark room at an average temperature of 26.5°C. and compared with 9 litter-mates of the same sex kept in ordinary room light at 25° and irradiated an average of 40 minutes weekly at 24 inches with an Alpine sun (Mercury vapor) lamp. During the last 75 days of this study 3 rats of about 60 gm. and 6 uncontrolled young, born in the dark, were added to the series. The diet (yellow corn 62; linseed oil meal 13; whole milk powder 18; casein 4; alfalfa 2; sodium chloride 0.5; and calcium carbonate 0.5) had an iodine content of 13.5 gamma/100 gm. (second month) and 15.6 gamma/100 gm. (third month).