Urine iodine concentration in hospitalised infants with thyroid dysfunction.

Background: Iodine is essential to thyroid hormone production, and both excess and deficiency can cause thyroid dysfunction in infants. While urinary iodine concentration (UIC) is used to assess population iodine status, there is no gold standard for determining iodine status in individual infants. Our study aimed to examine the clinical use of UIC in the investigation of thyroid dysfunction in hospitalised infants.

Methods: We examined hospital records of infants (age < 24 months) admitted to The Children's Hospital at Westmead who had UIC collected in the context of thyroid dysfunction between 2007-2009 and 2017-2021, two time periods separated by changes in public health measures for iodine nutrition and local clinical practice.

Results: Of 152 infants, 13.8% had UIC in iodine deficient range (WHO population-based definition: UIC < 100 µg/L) and 53.9% in iodine excess range (UIC ≥ 300 µg/L). Highest quartile UIC (> 1432 µg/L) was significantly associated with pre-test clinician suspicion of iodine excess, identification of source of iodine exposure, higher percentage of premature babies, and those with cardiac anomalies or who required surgery. Median free thyroxine (fT4) level was significantly lower in the highest UIC quartile group compared to the lower three quartiles (9.4pmol/L [interquartile range 7.8-vs 13.7] vs. 12.7 pmol/L [10.3-15.6]; p = 0.004). While median TSH was elevated in all UIC quartiles in this group, there were no significant differences in the levels between the UIC quartile groups.

Conclusions: Extremely high random UIC can be helpful to confirm clinical suspicion of iodine excess in hospital-based infants, taken in the context of thyroid dysfunction in critical illness. The degree of thyroid dysfunction associated with high UIC in this clinically complex and often premature patient population may be better measured by the fT4 level rather than the degree of TSH elevation.