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. So far, it was assumed that the colour sensitivity of the cones is fixed in the adult retina.

Martin Glösmann and Anika Glaschke in Leo Peichl’s team at the Max Planck Institute for Brain Research, Frankfurt, Germany, and their colleagues at the universities of Frankfurt and Vienna, Austria, showed 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, e.g., as a consequence of dietary iodine deficiency or removal of the thyroid, would affect 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.

Image (C) Wiley-VCH

• Max Plank Institute, Munich, Germany