Which antithyroid agent is used in the initial management of thyroid storm?

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In thyroid storm the priority is to stop new thyroid hormone from being made and to reduce circulating hormone quickly. The drugs that accomplish this are thionamides, specifically propylthiouracil or methimazole. They inhibit thyroid peroxidase, blocking iodination and coupling of tyrosine residues so that less thyroid hormone is synthesized. Propylthiouracil has the additional effect of decreasing peripheral conversion of T4 to the more active T3, which helps speed relief of the hypermetabolic state; methimazole is a very effective alternative for inhibiting synthesis, and is used when PTU isn’t preferred or available. After starting one of these, other measures—such as beta-blockade for adrenergic symptoms and glucocorticoids for potential adrenal support and further reduction of T4 to T3—are added, with iodine given after thionamides to suppress release. The key point is that the initial antithyroid therapy in thyroid storm is a thionamide, either propylthiouracil or methimazole.