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.
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.
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.
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), and 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.