Human fetal thyroid cell growth in vitro: System characterization and cytokine inhibition

We have studied primary human fetal thyroid cell monolayers as an in vitro model for thyroid cell growth and function. Fetal thyroid cells grew slowly in a serum-free medium under basal conditions (i.e. without insulin and TSH), with a doubling time of 112 ± 7 h (mean ± SEM), indicating autonomous growth capacity. Addition of insulin (10 μg/ml) lead to increased growth, with a doubling time of 43.0 ± 2.5 h, and TSH further reduced the doubling time in a dose-dependent manner, with the highest growth rate at 1-10 mU/ml (doubling time, 27 ± 0.5 h). These growth rates were only observed when cells were subjected to a short culture time (12 h) before investigation, whereas after 96 h of culture the fetal thyroid cell growth rate was reduced by up to 50%. Addition of more than 5% serum completely inhibited the growth stimulation initiated by insulin and TSH. The accumulation of extracellular cAMP by the fetal cell monolayer was induced by TSH in a dose-dependent manner and reached a maximum effect at 10 mU/ml (4.8 ± 0.6 pmol cAMP). Basal thyroglobulin (Tg) release was 26.9 ± 0.5 ng/10⁵ cells·day. Insulin decreased Tg release to 16.7 ± 0.5 ng/10⁵ cells· day, whereas TSH increased it up to 52.5 ± 1.0 ng/10⁵ cells·day. The T cell cytokine γ-interferon, a product of the lymphocytic infiltrate in autoimmune thyroid disease, significantly reduced both insulin and TSH-stimulated cellular growth as well as accumulation of Tg. In conclusion, human fetal thyroid cell monolayers grew at high velocity under defined experimental conditions in vitro. These conditions included a low serum concentration, short culture time before investigation, and insulin/TSH supplementation. Furthermore, this human thyroid model confirms our earlier observations on the influence of cytokines in thyroid cell growth regulation.