What part does the thyroid gland have in vision? Thyroid hormone is crucially involved in controlling which visual pigment is produced in the cones. Previously, it was assumed that the colour sensitivity of the cones is fixed in the adult retina. Researchers at the Max Planck Institute for Brain Research in Frankfurt/M., together with colleagues at the University of Frankfurt and universities in Vienna, have now been able to show that in mature cones of mice and rats the production of visual pigment is regulated by thyroid hormone. It is assumed that this mechanism exists in all mammals, including humans. If so, the adult-onset of thyroid hormone deficiency would affect colour vision.
Thyroid hormone has a crucial role during development of the body and also of the nervous system. Children born with a thyroid hormone deficiency have serious defects of physiological and mental development, hence newborns are routinely checked for thyroid hormone deficiency, and hormone substitution therapy is given when indicated.
Studies in mice have shown that thyroid hormone also plays an important role in the development of the eye and particularly the cone visual cells. In the retina of the eye, the cones are the visual cells responsible for colour vision. Most mammals have two spectral cone types containing either of two visual pigments (opsins), one sensitive to shortwave light (UV/blue opsin), the other to middle-to-longwave light (green opsin). Cones express a thyroid hormone receptor. Its activation by the hormone suppresses the synthesis of UV/blue opsin and activates the production of green opsin.
"In addition to their importance for basic retinal research, our findings may also have clinical relevance", says Martin Glösmann, who currently examines the genetic foundations of the process at the University of Veterinary Medicine, Vienna. "If this mechanism also acts in human cones, the adult-onset of thyroid hormone deficiency - e.g. as a consequence of dietary iodine deficiency or removal of the thyroid - would also affect the cone opsins and colour vision". There are no such reports in the clinical literature, presumably because the general symptoms of thyroid hormone deficiency are so severe that therapy is initiated before the cone opsin shifts would show up.