It has been suggested that depressed thyroid function in infants with RDS is secondary to their respiratory problem. This study was conducted to assess the patterns of change in thyroid hormones in premature infants with and without RDS, and to clarify the role of prematurity versus the effect of illness on these changes. METHOD: Serum TSH, T4, FT4 and T3 concentrations were measured by radioimmunoassay in 18 healthy premature infants and 13 premature infants with RDS. Blood samples were collected at birth and at 12-24hrs, 5days and 15days after delivery. RESULTS: 1) Both mean gestational age and birth weight values for premature infants with RDS were not significantly different from those for healthy premature infants. 2) At birth, there were no significant differences in serum TSH, T4, FT4 and T3 concentrations between healthy premature infants and premature infants with RDS(p>0.05). 3) At 12-24hrs of age, premature infants with RDS had significantly premature infants. 4) At 5 and 15days of age, there were no significant differences in serum levles of TSH, T4, FT4, and T3 between healthy premature infants and premature infants with RDS(p>0.05). CONCLUSIONS: The pattern of decreasing thyroid function in premature infants with RDS resembles the pattern of change in ill adult patients, which suggests that these variations are due to illness.