Your thyroid is a gland shaped somewhat like a butterfly whose "wings" (thyroid lobes) are wrapped around the trachea in your neck.
Your thyroid gland works together with your hypothalamus (part of your brain) and your pituitary gland to affect the metabolism of organs like your heart, liver and muscles.
When your thyroid doesn't produce enough hormones, this is called hypothyroidism. When your thyroid produces too much hormones, this is hyperthyroidism.
Your eyes are very much affected by your thyroid gland, going back as far as the womb. Your thyroid is instrumental in the development of your eyes, especially in your retinas' cone visual cells.
Your retina is in the back of your eye. When images go through the lens, they are focused on the retina. Your retina turns the images into electric signals. These signals are sent to the brain by way of the optic nerve.
Cone visual cells are visual receptor cells in the retina that are sensitive to color and to bright light. They determine which color you see.
We have two spectral cone types which contain opsins (visual pigments). UV/blue opsin responds to short wave light. Green opsin responds to middle wave and long wave light.
Thyroid hormone receptors coming from the cones can reduce usage of UV/blue opsin and initiate an increase of green opsin.
It was previously believed by scientists that once your eyes had developed and your own particular opsin combination was in place, things were permanently settled. But now some scientists are questioning this theory.
Researchers from the Max Planck Institute for Brain Research in Frankfurt Germany and the University of Frankfurt, as well as universities in Vienna, seem to have developed a different picture.
It would appear that the amount of thyroid hormone you're producing can have an ongoing effect on your ability to see in color.
The study was performed on rats and mice, on the assumption that as mammals, the research results concerning rats would translate into something usable for humans.