BACKGROUND: Thyroid function in asphyxiated newborns who received hypothermia therapy and its relation to neurological outcome are not well described. METHODS: We performed a prospective study to measure thyroid function in 12 asphyxiated newborns who received hypothermia therapy. We measured serum thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) on admission, at 24, 72, and 96 h after birth, and at discharge (range, 17-54 days). The 12 newborns were divided into two groups based on the presence of brain injury on head magnetic resonance imaging (six in the abnormal imaging group and six in the normal imaging group), and thyroid function was compared between the two groups. RESULTS: Serum TSH was within the normal range in the 12 newborns. Serum FT3 and FT4 remained low at 24, 72, and 96 h after birth, and returned to normal range at discharge in the 12 newborns. There was no significant difference in serum TSH between the two groups, but serum FT3 at 96 h after birth, and serum FT4 at 72 and 96 h after birth, were significantly lower in the abnormal imaging group than in the normal imaging group (P = 0.02; P = 0.03; and P = 0.01, respectively). CONCLUSIONS: Asphyxiated newborns have transient low thyroid hormone levels at 24-96 h after birth. Serum FT3 and FT4 between 72 and 96 h after birth may predict brain injury in asphyxiated newborns.