OBJECTIVE To determine the frequency and characteristics of late rise of thyroid stimulating hormone (LRT) among ill newborns. METHODS Data were retrospectively analyzed from infants in intensive care settings with abnormal thyroid tests over 13 months. Thyroid tests were performed by filter paper if neonatal intensive care >4 weeks or serum if clinically indicated. LRT was defined as thyroid stimulating hormone (TSH) >10 microIU/ml after normal TSH on initial newborn screen. RESULTS LRT was identified in 13 infants. Of 736 admissions to the neonatal intensive care unit (NICU), 10 (1.4%) had LRT. Excluding 3/10 with diagnosis at <1 week of age the frequency is 0.95%. Three additional cases occurred in other ICUs. TSH elevation resolved in 6/13 (group A, TSH 10.6-20.6 microIU/ml) and persisted in 7/13 necessitating treatment (group B, TSH 10.5-1326 microIU/ml). 7/13 had birth weights <1500 g. 11/13 had gestational ages <37 weeks. LRT was associated with surgery, sepsis workup, dopamine, and gastrointestinal disorders. CONCLUSIONS LRT was not infrequent in ill newborns. Most were premature and half were not very low birth weight. We recommend monitoring of thyroid function by serum specimen in ill newborns with prolonged ICU care regardless of birth weight.