Context. Terminal differentiation of the human thyroid is characterized by the onset of follicle formation and thyroid hormone synthesis at 11 gestational weeks (GW).

Objective. This study aimed to investigate the ontogeny of thyroglobulin (Tg), thyroid peroxidase (TPO), sodium/iodide symporter (NIS), Pendred syndrome gene (PDS), Dual-oxidase 2 (DUOX2), TSH-receptor (TSHR), and TITF1, FOXE1 and PAX8 in the developing human thyroid.

Design. Thyroid tissues from human embryos and fetuses (7-33 GW; n = 45) were analyzed by quantitative PCR to monitor mRNA expression for each gene, and by immunohistochemistry to determine the cellular distribution of TITF1, TSHR, Tg, TPO, NIS, and the onset of thyroxine (T4) production. A broken line regression model was fitted for each gene to compare the loglinear increase in expression before and after the onset of T4 synthesis.

Results. TITF1, FOXE1, PAX8, TSHR, and DUOX2 were stably expressed from 7 to 33 GW. Tg, TPO, and PDS expression was detectable as early as 7 GW, was correlated with gestational age (all, P < 0.01), and the slope of the regression line was significantly different before and after the onset of T4 synthesis at 11 GW (all, P < 0.01). NIS expression appeared last, and showed the highest fit by the broken line regression model of all genes (correlation age P < 0.0001, broken line regression P < 0.0001). Immunohistochemical studies detected TITF1, TSHR, and Tg in unpolarized thyrocytes before follicle formation. T4 and NIS labeling were only found in developing follicles from 11 GW on.

Conclusion. These results imply a key role of NIS for the onset of human thyroid function.