Atypical hypothyroidism and the very low birthweight infant

Results of thyroid screening tests were examined retrospectively on 311,282 infants born in Massachusetts from January 1, 1993 to December 31, 1996. D uring this period, 118 infants were found to have typical hypothyroidism, c haracterized by a low thyroxine (T-4) and an elevated thyrotropin (TSH) on the initial newborn-screening specimen. Of these, 98 were normal birthweigh t (NBW, greater than or equal to 2,500 g), 9 were low birthweight (LBW, 1,5 01-2,499 g), and 11 were very low birthweight (VLBW, less than or equal to 1,500 g). Atypical hypothyroidism as defined here is characterized by a low T-4 and normal TSH concentration on the initial screening specimen, follow ed by and elevated TSH level on a repeat blood specimen. This phenomenon oc curred in 18 infants, of whom 4 were NEW, 4 were LBW, and 10 were VLBW. The incidence of combined typical and atypical hypothyroidism was: NEW, 1:3051 ; LBW, 1:1589; VLBW, 1:153, with the highest incidence of atypical hypothyr oidism in the VLBW category (48% of cases in this weight category, 56% of a ll cases of atypical hypothyroidism). In addition, screening programs using a primary TSH screen will miss infants with atypical hypothyroidism In vie w of these results, it is suggested that T-4 measurements be obtained routi nely in all LBW and VLBW infants, with additional routine repeat blood spec imens.