IT IS A THIOUREA ANTITHYROID DRUG. IT ACTS AT 3 STEPS IN THE SYNTHESIS OF THYROID HORMONE, INHIBIT OXIDATION OF IODIDE, IODINATION OF TYROSINE RESIDUES, COUPLING OF IODOTYROSINE RESIDUES TO FORM T-3 AND T-4. IT DOES NOT INTERFERE WITH TRAPPING OF IODIDE AND DO NOT MODIFY THE ACTION OF T-3 AND T-4 ON PERIPHERAL TISSUES OR ON PITUITARY. GOITER IS NOT THE RESULT OF POTENTIATION OF TSH ACTION ON THYROID, BUT IS DUE TO INCREASED TSH RELEASE AS A CONSEQUENCE OF REDUCTION IN FEED BACK INHIBITION.