Thyroglobulin gene expression is regulated by insulin and insulin-like growth factor I, as well as thyrotropin, in FRTL-5 thyroid cells.

In FRTL-5 thyroid cells depleted of thyrotropin, insulin, and serum for 3 days, growth and synthesis of thyroglobulin are inhibited. Readdition of insulin or IGF-I in the absence of thyrotropin stimulates thyroglobulin synthesis or thyroglobulin mRNA level by 2-4-fold but has no effect on cell growth. The half-maximal effects of insulin and IGF-I are at 100 and 10 ng/ml, respectively. In both cases, the increased mRNA levels are accompanied by an increase in transcription rate. Maximally effective concentrations of insulin or IGF-I are additive with thyrotropin in increasing thyroglobulin mRNA levels and in increasing transcription; in contrast, the effect of insulin is not additive with IGF-I.