THYROTROPIN-RELEASING-HORMONE STIMULATION TESTS IN INFANTS

The TSH response to TRH administration (7 mug/kg) was measured in 68 i nfants (22 premature) who had abnormal thyroid screening tests by the filter paper method and whose serum thyroid function tests were only m ildly abnormal. Twenty-eight infants (12 premature) had peak TSH value s of 35 mU/L or less and were considered normal (group I). Forty infan ts (10 premature) had peak TSH values above 35 mU/L and were considere d hyperresponsive (group II). The mean age at testing, screening T4, T SH levels that prompted the testing, as well as baseline T4, T3, and f ree T4 at the time of TRH testing were not different between the group s. The mean (+/- SD) baseline TSH value was greater in group II (6.8 /- 2.3 mU/L) than in group I (4.4 +/- 2.2 mU/L; P < 0.001). However, t here was a great deal of overlap in the individual TSH values (group I , 0.9-10 mU/L; group II, 1.9-10.6 mU/L). Mean peak TSH levels were sig nificantly different in the two groups (group I, 24 +/- 7.7 mU/L; grou p II, 60.3 +/- 26.1 mU/L; P < 0.001). During long term follow-up, all 25 group I infants available for evaluation have been confirmed as cli nically and biochemically normal. No infant diagnosed as normal was la ter found to have evidence of hypothyroidism. Fourteen infants in grou p II have had evidence of thyroid dysfunction. We conclude that the TS H response to TRH stimulation is a useful tool for the evaluation of i nfants suspected of having primary hypothyroidism. Whether hyperrespon siveness to TRH represents a form of neonatal hypothyroidism requiring treatment remains to be determined.