Effect of theophylline on thyroid status in the rat

The effect of administration of theophylline, thyroxine (T4), or a combination of the two, for 14 days on body weight, food intake, serum concentrations of T4 and 3,5,3'-triiodothyronine (T3), and on the rate of 5'-monodieodination of T4 to T3 by hepatic and renal homogenates was evaluated. Administration of T4 (25 micrograms/kg b.w./day) increased the serum concentration of T4 and T4 5'-deiodinase activity. Administration of theophylline (191 mg/kg b.w./day) increased the serum concentration of theophylline to 28.5 micrograms/ml, increased food intake, but decreased both the concentration of T4 in serum and also T4 5'-deiodinase activity. Administration of a combination of T4 and theophylline returned T4 5'-deiodinase activity to control levels. Since the addition of only high concentrations of theophylline to incubation mixtures inhibited T4 5'-deiodinase directly, it is most likely that the effect of theophylline in vivo on the activity of the T4 5'-deiodinase enzyme is indirect and secondary to inhibition of thyroid hormone secretion.