Light and Thyroid Function

Abstract

The close connection between the functions of the gonads and the thyroid in their effect on growth, reproduction, molting, and bird migration prompted experiments concerning the influence of varying light conditions on thyroid function. Schildmacher (1938) discovered that the thyroid colloid of the male garden redstart evidenced absorption in the fall (September to December). The degree of absorption was not noticeably influenced by additional exposure to light or the injection of male sex hormones. On the other hand, the injection of thyroxine was very effective; thyroid glands in a storage state with an extremely flat epithelium were formed. Exposure to artificially lengthened days from March to April caused molting and weight increase in castrated male green finches; the thyroid’s advanced state of absorption indicates the organ’s heightened activity (Rautenberg 1952). Artificially shortened days in summer do not affect the annual rhythm of thyroid activity in green finches. Exposure to lengthened days, however, increases the production of the thyrotropic (TSH) hormone (Schildmacher 1956).