As in humans and mice with congenital hypothyroidism with mutant Tg, T4-containing protein — while dramatically diminished relative to that found in WT thyroid follicles — was apparent in the lumen of 89.7% ± 13.3% (SD, n = 5 animals) of the thyroid follicles of untreated TGrdw/rdw rats (Figure 4), surrounded by a monolayer of Pax8-positive cells (indicative of thyrocytes, Figure 4A). Here, TG is linked to congenital hypothyroidism.