Effects of vitamin A deficiency on thyroid function and serum thyroxine levels in the rat.

The effects of vitamin A deficiency, which results in a substantial decrease in the level of serum retinol binding protein, on the existent state of serum thyroxine and thyroid function were examined. In the vitamin A-deficient rats, the thyroid weight increased and the level of serum thyroxine decreased to one half that of the control rats. Normal thyroid weight and serum thyroxine levels were recovered by the replenishment of retinyl palmitate in the vitamin A-deficient rats. In addition, decreased hormone synthesis was observed in the thyroid glands of the vitamin A-deficient rats. The determination of thyroxine distribution in rat serum proteins in vivo showed that thyroxine-binding prealbumin (TBPA) is a major thyroxine transport protein in the control rats, whereas in the deficient rats the amount of thyroxine bound to TBPA decreased and the thyroxine bound to thyroxine-binding globulin (TBG) increased significantly as compared with observations in control rats. These findings suggest that the existent state of serum thyroxine and thyroid function is affected by the serum level of vitamin A.