The AAP has no current recommendations regarding the management of infants with borderline abnormal newborn thyroid screening who continue to have borderline results on confirmatory tests. We determined the percentage of patients with borderline hypothyroidism in infancy who had persistent hypothyroidism after 3 years.

Chart review from 1988 onwards identified fifteen fullterm infants (10 males, 5 females) who had borderline abnormal results on screening (total thyroxine less than 10th percentile, TSH <40 uU/ml). Twelve of the 15 subjects had a modestly elevated TSH on serum confirmatory testing (6.0-18.8 uU/ml). Three patients with normal TSH on repeat serum confirmatory tests had an abnormal TSH response to TRH testing (TSH peak >35 uU/ml). All fifteen were treated with L-thyroxine. All were available for reevaluation after 3 years when thyroxine was discontinued on 14 of the 15 patients. One infant remained on L-thyroxine because of an elevated random TSH (10 uU/ml). All patients had thyroid function tests one month after discontinuing L-thyroxine.

Nine patients were restarted on L-thyroxine because of an elevated basal TSH (mean 7.29 uU/ml, normal 0.4-4.6). One patient was restarted because of low free T4(0.7 ng/dl, normal 0.8-2) with a TSH in the upper limit of normal(4.13 uU/ml). The remaining four infants underwent TRH testing 3 months later. There were 3 TRH hyperresponders consistent with primary hypothyroidism(mean TSH peak 44.1 uU/ml). One patient had a normal response (peak TSH 12.2 uU/ml).

Only one of our fifteen subjects with borderline TSH elevations in infancy had normal thyroxine, basal TSH and TSH response to TRH at 3 years of age. Whether infants with “mild hypothyroidism” or “compensated hypothyroidism” benefit from thyroid hormone replacement in infancy is not known. Based on our findings, 93% of these patients (14 of 15) continued to have abnormal thyroid function studies at 3 years of age. It would seem prudent to start thyroxine in infancy when borderline abnormal results of newborn screening persist on subsequent serum confirmatory tests.