Late-onset circulatory dysfunction after thyroid hormone treatment in an extremely low birth weight infant

Late-onset circulatory dysfunction (LCD) is a phenomenon specific to premature infants and is characterized by sudden onset of hyponatremia, hypotension, oliguria and non-physiological weight gain, without an obvious cause, in premature infants after stabilization of circulation and respiration. The cause of LCD is not clear, but adrenal insufficiency in premature infants is a severe syndrome because steroid replacement therapy is often essential to treat the symptoms. We report a rare case of a premature infant who developed an LCD crisis the day after thyroxine replacement therapy. The female infant was born at 25 weeks of gestational age, weighing 672 g, and appeared to have hypothyroidism, with free T4 of 0.19 ng/dl and elevated TSH levels of 26.3 microIU/ml at Day 14. She developed an LCD crisis the day after starting thyroxine treatment. She received steroid replacement therapy for 4 weeks and her adrenal function progressively recovered. She also needed thyroxine supplementation for 13 weeks, which maintained her thyroid function as euthyroid. Because she exhibited cortisol insufficiency and thyroid hormone insufficiency, the antecedent thyroid hormone replacement may be responsible for the onset of LCD. We must consider monitoring adrenal function when starting thyroxine therapy in premature infants with hypothyroxinemia.