FOLLOW-UP OF NEWBORNS WITH LOW THYROXINE AND NONELEVATED THYROID-STIMULATING HORMONE-SCREENING CONCENTRATIONS - RESULTS OF THE 20-YEAR EXPERIENCE IN THE NORTHWEST REGIONAL NEWBORN SCREENING-PROGRAM

Objectives: To determine the type and frequency of thyroid disorders d etected in infants with low thyroxine (T4) and nonelevated thyroid-sti mulating hormone (TSH) screening test results in the Northwest regiona l Newborn Screening Program (NWRNSP) over the 20-year period from Mag 1975 to Mag 1995 and to determine the effect of follow-up of these inf ants on the overall recall fate. Study design: The NWRNSP requests a s erum specimen in infants with an absolute T4 level < 38.6 nmol/L <3 mg /dl) and in infants with two filter paper T4 concentrations less than the 3%, regardless of the TSH concentration. We conducted a retrospect ive analysis of infants who were followed up because of low T4 and non elevated TSH concentrations on newborn screening. To determine the eff ect of follow-up of infants with lo Lv T4 levels, nonelevated TSH conc entrations on the recall rate,we selected 1 ear (1994) far review Seru m sample requests were evaluated to determine the reason for the reque st. Results: Over this 20-year period, the NWRNSP detected 450 infants with primary hypothyroidism among 1,747,805 infants screened (1:3,884 ). Of these, 416 were detected on the basis of low T4 levels and nonel evated TSH screening test results, whereas an additional 34 infants wi th primary hypothyroidism and 29 infants with hypopituitary hypothyroi dism were detected as a result of followup of low T4 levels and nonele vated TSI-I screening test results. This included 25 infants with dela yed TSH rise (1:67,226), 9 infants with mild hypothyroidism (TSH level s <25 mU/L) (1:194,212), 29 infants with hypopituitary hypothyroidism (1:60,269)1 and 434 infants with T4-binding globulin deficiency (1:4,0 27). Excluding those with T4-ginding globulin deficiency, the false-po sitive rate was 43.5:1. This compares with an overall false-positive r ate of 12:1 for our screening program. Conclusion: Follow-up of infant s with lo T4 and nonelevated TSH concentration on screening led to the detection of 63 additional infants with hypothyroidism, for an overal l frequency of 1:27.743. We believe this yield justifies continued fol low-up of infants with low T4 levels, nonelevated (TSH) screening rest results in our program.