BACKGROUND Postpartum thyroiditis is a common but frequently unrecognized disorder, affecting approximately 5% of women during the first 12 months after delivery. We investigated whether the antimicrosomal antibody titer could be used to determine which women with positive titers postpartum (1) might develop symptomatic or biochemical abnormalities within the first postpartum year (early disease), (2) might require therapy with thyroid hormone, and (3) might have persistent abnormalities (late disease). METHODS Women (n = 55) who had positive antimicrosomal antibody titers at delivery were prospectively followed for 11 to 45 months. Titers were evaluated again at 6 to 10 weeks postpartum and approximately every 8 weeks for the first year. RESULTS Early disease occurred in 40 of 55 (73%) women, late disease occurred in 29 of 55 (53%) women, and treatment was required by 21 of 55 (38%) women. The occurrence of early disease was associated with the occurrence of late disease (P < .05). The chances of developing early disease were 6 to 1 (P = .01) when serum titers of antimicrosomal antibodies were > or = 400 at delivery, and 5 to 1 (P = .02) when titers were > or = 1600 at 6 to 10 weeks postpartum. The chances of being given thyroid hormone therapy were 23 to 1 (P = .006) when titers at delivery were > or = 6400, and 6 to 1 when titers at 6 to 10 weeks postpartum were > or = 6400 (P = .004). Titers were not useful in estimating who would have late disease. CONCLUSIONS Screening for postpartum thyroid dysfunction after delivery using antimicrosomal antibody titers is highly useful. The titer value can help guide the physician in the care of patients with postpartum thyroiditis whose disease may not be self-limiting and who will probably require thyroid hormone therapy.