In the present study, we used FRTL-5 cells to study the effects of fibroblast growth factors (FGFs) on 5'-deiodinase (5'D) activity and messenger RNA (mRNA) levels. In FRTL-5 cells deprived of TSH for 7 days, type I 5'-deiodinase (5'D-I) activity decreased to low, but detectable levels. Incubation of cells with acidic and basic FGFs significantly decreased 5'D-I activity below the basal levels. After 7 days of TSH deprivation, the addition of TSH (100 microU/ml) to the medium for 3 days resulted in an increase in 5'D-I activity. This TSH-induced increase in 5'D-I activity was blocked by the FGFs in a dose-dependent manner. Kinetic analysis revealed that both acidic and basic FGFs decreased the maximum velocity of 5'D-I activity in the presence or absence of TSH, without any significant effect on the Km of enzyme binding. HPLC analysis of the products of the 5'D-I assay revealed that there was no sequential deiodination of rT3. Incubation of FRTL-5 cells with acidic or basic FGF did not affect basal cAMP concentrations, nor did they block the TSH-induced rise in cAMP. However, acidic and basic FGFs inhibited forskolin- and (Bu)2cAMP-induced increases in 5'D-I activity. Incubation of FRTL-5 cells with TSH, (Bu)2cAMP, and forskolin increased 5'D-I mRNA levels. Incubation of FRTL-5 cells with acidic and basic FGFs decreased steady state 5'D-I mRNA levels and blocked the TSH-, forskolin-, and (Bu)2cAMP-induced increases in 5'D-I mRNA. In conclusion, we have demonstrated that FGFs inhibit 5'D-I activity and mRNA levels in FRTL-5 cells in the presence or absence of TSH. The inhibitory effect of FGFs on 5'D-I in FRTL-5 cells is mediated through either a cAMP-independent pathway or pathways distal to the generation of cAMP. The present data together with the identification of FGF in the thyroid gland suggest that FGF may play a physiological role in the regulation of thyroid hormone secretion.