Some Effects of 2,4-Dichlorophenoxyacetic Acid on Thyroid Function in the Rat: Effects on Iodine Accumulation1

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Abstract
2,4-Dichlorophenoxyacetic acid, a compound widely used as a weedicide, markedly increases I131 uptake by rat thyroid in vivo. Measurements of a number of thyroid function criteria, including thyroid cell height, rate of discharge of I131 from the gland, and serum and pituitary TSH levels, indicate that this effect does not involve the pituitary thyrotrophic hormone but represents a direct effect on the peripheral iodide pool. This was confirmed by isotope equilibrium analysis of the serum iodide level. The effect can be demonstrated only in rats in which the thyroid functions normally, and does not occur in hypophysectomized or iodine-depleted animals.