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

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Abstract
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 1/2 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. 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; in the deficient rats the amount of thyroxine bound to TBPA decreased and the thyroxine bound to thyroxine-binding globulin (TBG) increased significantly compared with observations in control rats. The existent state of serum thyroxine and thyroid function apparently is affected by the serum level of vitamin A.