Theophylline increases the uptake of radioiodine by mouse thyroid

Diagnostic and therapeutic use of radioiodine in the management of thyroid disorders depends on the ability of thyroid cells to concentrate radioiodine, a process that is regulated by the intracellular increase in cAMP. We hypothesized that theophylline, a drug known to increase intracellular cAMP via inhibition of phosphodiesterase, could increase thyroidal radioiodine uptake. We tested this effect in vivo, using C57BL/6j mice, and in vitro, using Fisher rat thyroid (FRTL-5) cells. One mouse received 2.5 mg theophylline i.p., whereas a control mouse received only saline. Twenty-hours after theophylline, mice were injected with 10 μCi Na¹²⁵I in 0.1 mL saline through the tail vein. Mean thyroidal ¹²⁵I activity was 3.3-fold higher in theophylline-treated mice than in their respective controls. Radioiodine uptake and intracellular cAMP production of FRTL-5 cells were increased by a relatively low concentration of theophylline (1 μM). Intracellular cAMP increased up to 30 min and then declined in response to 1 μM theophylline. Sera from theophylline-treated mice stimulated ¹²⁵I uptake and intracellular cAMP production by FRTL-5 cells. These findings show that theophylline can enhance radioiodine uptake by thyrocytes in vivo and in vitro. The in vitro effects of theophylline on both radioiodine uptake and cAMP production in a dose-dependent manner are consistent with an action mediated by phosphodiesterase inhibition.