In the absence of transcription factor TTF1, the thyroid gland, including the C-cells that produce calcitonin, will not form, while the absence of transcription factor PAX8 will result in thyroid agenesis, resulting in athyreotic Pax8−/− pups dying within a few weeks of birth [5,6,7,9,10]. Here, PAX8 is linked to hypothyroidism, congenital, nongoitrous, 2.