Congenital hypothyroidism (CH) occurs due to dysgenesis or dyshormonogenesis of the thyroid gland. Newborn screening for CH was introduced in the UK over 30 years ago and has almost eliminated the severe intellectual deficits caused by the deficiency of thyroxine to the developing brain. The recognised incidence of CH increased immediately post introduction of screening due to the improved detection and diagnosis of cases. However, further increases in the incidence of CH have been reported internationally and this is variably suggested to be due to a combination of lower screening detection thresholds, changes in population demographics and iodine status. Using data from over 1700 infants who have been referred to a single centre with positive CH screening results, lessons have been learnt in terms of the impact of changing screening TSH thresholds, underlying the physiology and genetics of CH and outcome data. In this cohort, we demonstrated that the group of infants classified as Asian/British Asian and Chinese by the UK Office of National Statistics have higher TSH cut points than the group of infants classified as white. This Asian group is also over-represented in the cohort referred from the CH screening laboratory compared to the background population. In addition, there is a high incidence of genetic mutations in the DUOX2 pathway for infants with borderline screening results. These mutations are reported most frequently in populations from the Asian subcontinent. They may be associated with transient CH. Using data from this same cohort, we have studied audiology outcomes. These suggest that there in an increase in hearing loss in infants with CH, and this is not detected by newborn hearing screening. In summary, 30 years after the introduction of newborn CH screening, there are still many unanswered questions regarding the physiology, genetics and outcomes of infants with CH.