Vitamin A (retinol) and its biologically active derivatives, such as all-trans-retinoic acid (RA) and 9-cis RA, collectively called retinoids, have profound effects on vision, reproduction, growth and differentiation of organisms, as well as on embryonal morphogenesis (1).
Vitamin A is a liposoluble nutrient derived from retinyl esters in animal fat and carotinoids in plants, which is transported in the plasma bound to retinol-binding protein and, once entered into the cells, serves as the precursor of the biologically active RA by means of intracellular oxidation. This process involves cellular retinol-binding proteins and the enzymes retinol- and retinal-dehydrogenase. Probably, intracellular isomerases further convert part of RA to 9-cis-RA (2).
The relationship between retinoids and thyroid function was suspected long ago when an orange–yellow pigmentation of the skin was constantly described in long-standing hypothyroid patients. This colouring of the skin was later demonstrated to be due to