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Although newborn screening (NBS) for congenital hypothyroidism (CH) in Japan started more than 40 years ago, the prevalence of CH remains unclear. Prevalence estimations among NBS-positive CH individuals include those with transient hypothyroidism and transient hyperthyrotropinemia, and re-evaluation with increasing age is necessary to clarify the actual incidence. Thus, we re-evaluated the incidence of permanent CH. Of the 106,114 patients who underwent NBS in the Niigata Prefecture, Japan, between April 2002 and March 2006, 116 were examined further due to high thyroid-stimulating hormone levels (>8 mIU/L) and were included in the study. We retrospectively evaluated their levothyroxine sodium (LT4) replacement therapy status from the first visit to 15 years of age. Of the 116 NBS-positive patients, 105 (91%) were initially examined in our department. Of these, 72 (69%) started LT4 replacement therapy on the first visit. Subsequently, 27 patients continued LT4 replacement until 15 years of age after multiple re-evaluations. The prevalence of permanent CH in the Niigata Prefecture during this period was 1 in 2500–3500 children. Ultimately, 62.5% of patients on LT4 replacement discontinued treatment by 15 years of age. This is the first study to clarify the true prevalence of permanent CH in Japan.

Newborn screening (NBS) can detect 21-hydroxylase deficiency (21-OHD), allowing for early treatment initiation. However, many patients present with adrenal crises or hyponatremia at their first visit. Age (in days) of hyponatremia development in infants with salt-wasting (SW)-type 21-OHD remains unclear. Therefore, we determined the earliest age of hyponatremia diagnosis in this retrospective observational study using medical records of 40 patients with classic 21-OHD in Niigata Prefecture, Japan, from April 1989 to March 2019. We determined the earliest diagnosis of hyponatremia (serum sodium levels < 130 mEq/L) and created a sodium decrease rate model to estimate hyponatremia development age. Of 23 patients with SW-type 21-OHD, 10 (43.5%) were identified during NBS; the earliest case to present with hyponatremia was at day 7. Serum sodium levels were significantly and negatively correlated with age in days, and hyponatremia was estimated to develop at 6.6 d after birth. Genotype or serum 17-hydroxyprogesterone levels were not associated with sodium decrease rate. Thus, hyponatremia development age is earlier (within 7 d) than the previously described time-point (10-14 d) in infants with SW-type 21-OHD. Efforts to reduce the time lag from obtaining results to consultation may be required in patients with high 17-hydroxyprogesterone levels on NBS.

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