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 μU/ml during the recovery phase when ¹³¹I uptake was normal or hyper-normal. In addition, and 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.