Some Effects of 2,4-Dichlorophenoxyacetic Acid on Thyroid Function in the Rat: Effects on Peripheral Thyroxine1

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Abstract
2,4-Dichlorophenoxyacetic acid (2,4-D), a weedicide previously shown not to affect pituitary thyrotropin secretion, lowers the serum level of protein-bound iodine. The effect appears to be based upon a lowering of thyroxine binding by serum proteins from 2,4-D treated rats, which was demonstrated by dialysis, “thyroxine stabilization,” and in vitro incubation techniques. The thyroxine displaced from serum proteins localizes principally in the liver, and the apparent thyroxine distribution volume is significantly increased. The results are discussed with respect to the significance of tests for serum thyroxine binding and the role of serum thyroxine not bound to protein in controlling the pituitary-thyroid feedback system.