Thyroidal response following iodine excess for cardiac catheterisation and intervention in early infancy.

Background: Cardiac catheterisation requiring the use of contrast medium is increasingly utilised in infants with congenital heart disease. Thyroid function in infants is potentially vulnerable to relatively high doses of iodine.

Methods: Single-centre prospective study of 21 patients (10 neonates, 11 infants) exposed to iodine during cardiac catheterisation. Median age was 30days (1-180), median body weight 3.3kg (1.6-7.0). Serum-levels of thyroid-stimulating hormone (TSH), free triiodothyronine (fT3) and free thyroxine (fT4) were measured at baseline before, and at three different times points after intervention.

Results: Baseline thyroidal levels were within normal range in all patients. After applying contrast media of median 6.8ml/kg (range 4.5-14.9), we observed a wide range of TSH values: median 5.01μg/l (range 0.59-37.73; reference 0.72-11.0) three to five days after catheterisation. Five of the 10 neonates demonstrated transient TSH elevation consistent with latent hypothyroidism, and a sixth a transient drop in all hormone levels typical of non-thyroidal illness syndrome. However, circulating thyroid hormones remained within normal range, except for two borderline-low fT4 values within the first week after catheterisation that normalised without any medical intervention.

Conclusion: Systemic iodine exposure during cardiac catherterisation seems to be clinically well tolerated in early infancy. However, exposure to iodine has demonstrable but apparently reversible effects on thyroid hormones during a potentially important developmental period. The implications of this are unclear, but warrant further investigation in larger cohorts.