A β-blocker, such as metoprolol, is the most appropriate treatment for this patient with thyrotoxicosis who has subacute granulomatous (de Quervain) thyroiditis based on the low radioactive iodine uptake (RAIU) and the painful thyroid on examination. An antecedent event, in this case a viral illness, destroys the thyroid follicles and triggers the release of preformed thyroid hormones into the bloodstream, creating the thyrotoxic phase. During this phase, further release of thyroid hormones ceases, resulting in a low RAIU as seen in this patient. β-Blockers are beneficial in the thyrotoxic phase to block the adrenergic effects of the high circulating thyroid hormone levels. Treatment of subacute thyroiditis is otherwise typically supportive, with NSAIDs for pain as needed. In patients with severe pain, glucocorticoids may occasionally be used.

Blocking further release of thyroid hormones with a thionamide (either methimazole or propylthiouracil) is ineffective because the thyroid has already released preformed thyroid hormone into the bloodstream and is currently not producing or secreting additional thyroxine.

Administration of radioactive iodine for treatment of the hyperthyroidism is also ineffective because this patient's thyroid is not currently taking in iodine, as evidenced by the low uptake on his thyroid scan. More importantly, radioactive iodine will not treat this patient's underlying problem of damaged thyroid follicles and release of preformed thyroid hormone into the bloodstream.