Treatment plan for neonates with borderline hypothyroidism (persistent hyperthyrotropinemia with normothyroxinemia) has not been established. In this study, changes in thyroid function after discontinuation of low-dose L-thyroxine (L-T4) supplement in infants with the condition were evaluated. Fourteen infants with hyperthyrotropinemia at neonatal screening had repeated hyperthyrotropinemia (> 8 mU/L) with normothyroxinemia. TSH response was exaggerated at TRH testing. The subjects were treated with low-dose L-T4 (3 to 9 microg/kg/day) for 2.2 to 6 years, and euthyroid status was maintained. After discontinuation of therapy, mild hyperthyrotropinemia persisted up to 24 months, while serum FT4 remained within the lower half of the normal range. TSH response to TRH stimulation, which tended to be exaggerated 1 month after discontinuation, became lower 6 to 12 months later. RAIU and thyroid scintigraphy were normal in all subjects. No patient developed hypothyroxinemia, although mild elevation of TSH lasted rather long after discontinuation of low-dose L-T4 therapy. Administration of L-T4 was not resumed provided the subjects were followed at regular interval. Further long-term investigation is needed to define whether re-administration is necessary or not.