Sodium handling in congenitally hypothyroid neonates

Early observations emphasized the possible development of hyponatraemia in hypothyroid children and adults, but recently this has been questioned. Aim: To investigate whether hyponatraemia develops in hypothyroid status by examining sodium handling in screening-detected neonates and infants with congenital hypothyroidism (CH). Methods: Serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), sodium (Na), creatinine (Cr), urinary Na, Cr, fractional sodium excretion rate (FENa) and other chemicals were measured before and after L-thyroxine (LT4) replacement therapy in 32 screening-detected CH neonates (11M, 21F) and 16 age-matched control neonates. Results: No cases of hyponatraemia were found in the 32 CH neonates. Their serum Na concentrations (139.1+/-1.5 mmol/L, ranging from 136 to 142 mmol/L, median 139 mmol/L) were not statistically different from those of 16 control neonates (139.3+/-1.3 mmol/L, ranging from 137 to 142 mmol/L, median 139 mmol/L,). No correlation was found between serum levels of TSH and FT4 and serum Na or FENa. No significant changes were found in serum Na concentrations in hypothyroid neonates two months after LT4 replacement therapy. The serum Na concentration (139.1+/-0.3 mmol/L, n=25) before treatment did not change statistically (138.9+/-0.2 mmol/L, n=25) two months after LT4 replacement therapy. Conclusion: As seen in various earlier reports, hyponatraemia can occur in hypothyroid patients, but no causal relationship exists between them. When hyponatraemia is detected in hypothyroid children, it does not seem to be directly related to lack of thyroid hormones and therefore other possible causes should be sought.