Iodine insufficiency and neonatal hyperthyrotropinaemia in Hong Kong

23% of the neonatal hypothyroidism in Hong Kong is transient. The present study aims to evaluate iodine excretion in healthy pregnant women in Hong Kong and to determine whether iodine insufficiency may occur in the local population to account for the type of neonatal thyroid dysfunction seen in our screening programme.

Pilot screening of urinary iodine excretion was determined in 253 healthy pregnant women between 32 and 36 weeks gestation. Fetal and maternal thyroid function in relation to urinary iodine excretion was evaluated in another 55 pregnant women who had given birth to infants with cord blood TSH > or = 16 mIU/I (95th percentile of the cord blood TSH screening programme) and the results were compared to a control group of 160 healthy women whose infants had cord blood TSH < 16 mIU/I.

Using a cut-off value of 0.79 mumol/l, a level as defined by WHO as iodine deficiency, we found that 35.8% of the pregnant women had urinary iodine concentrations below this cut off value. We demonstrated that the existence of borderline iodine supply affected the maternal and fetal thyroid function as evidenced by (i) a negative correlation between maternal TSH and urinary iodine concentration, (ii) higher cord blood TSH in those infants whose mothers had a low urinary iodine concentration as compared to those in whose mothers it was normal, (iii) women who had given birth to infants with cord blood TSH > or = 16 mIU/I had lower urinary iodine concentrations and serum fT4 levels as compared to mothers who had given birth to infants with normal cord TSH levels, and their offspring also had higher cord blood thyroglobulin levels.

Although Hong Kong is not a goitrous area, borderline iodine deficiency exists. Iodization of salt in our community could obviate the necessity for iodine supplements in pregnant women and other at-risk groups.