Differences in screening and confirmatory thyroid profiles in congenital hypothyroid subtypes: implications for missed cases.

Subtypes of primary congenital hypothyroidism (CHT) showed different degrees of impairment of thyroid function at screening and confirmation. Patients with athyrosis showed the greatest impairment with mean value for screening 1-thyroxine (ScrT4) of 2.73 micrograms/dl (p < 0.0001 vs E group), screening thyroid-stimulating hormone (ScrTSH) of 327.6 microIU/ml (p < 0.0001 vs E group), confirmatory L-thyroxine (ConT4) of 1.21 micrograms/dl (p < 0.0001 vs E group), and confirmatory thyroid-stimulating hormone (ConTSH) of 572 microIU/ml (p < 0.0001 vs E group). Infants with dysgenetic ectopic thyroids (E group) showed the best preservation of function with mean values for ScrT4 of 6.0 micrograms/dl, ScrTSH of 229 microIU/ml, ConT4 of 6.23 micrograms/dl, and ConTSH of 324.8 microIU/ml. Generally, intermediate mean values were found for infants with normal or goitrous thyroids with values for ScrT4 of 4.26 micrograms/dl, ScrTSH of 205 microIU/ml, ConT4 of 3.21 micrograms/dl, and ConTSH of 428 microIU/ml. The risk for CHT being missed appeared to be greatest in patients with dysgenetic ectopic thyroids with the mean difference between ScrT4 and the L-thyroxine cutoff value (ScrT4Diff) being -2.2 micrograms/dl; five cases fell within 0.5 microgram/dl of the cutoff. The risk was least in infants with athyrosis with ScrT4Diff of -4.64 micrograms/dl (p < 0.0001 vs E group); no cases were within 1 microgram/dl of missing the cutoff. For infants with normal or goitrous thyroids ScrT4Diff was -3.44 micrograms/dl; one case was initially missed and three fell within 0.5 microgram/dl of the cutoff. The risk for missing CHT in screening is apparent. Clinical vigilance must be maintained to detect missed cases as early as possible.