Permissive role of thyrotropin on thyroid radioiodine uptake during the recovery phase of subacute thyroiditis.

Serial measurements of serum triiodothyronine (T3), thyroxine (T4), thyrotropin (TSH), and 4-hr thyroidal 131I uptake were carried out in nine patients with subacute thyroiditis. In the acute phase, suppressed TSH and 131I uptake were observed simultaneously with the elevations of T3 and T4. Thyrotropin-releasing hormone (TRH) failed to increase TSH in all patients studied. The mean value of an increment in serum TSH was only 1.8 muU/ml during the recovery phase when 131I uptake was normal or hypernormal. In addition, an elevated 131I uptake was not necessarily associated with an immediate increase in the serum T3 and T4. These observations suggest that the resumption of the iodide pump may be more important than an increment in TSH in producing normal or hypernormal 131I uptake during the recovery phase. There appears to be a dissociation between the reestablishment of 131I uptake and the resumption of the mechanism of hormonal synthesis and secretion in the thyroid.