OBJECTIVE To study thyroid function in children infected with human immunodeficiency virus (HIV) and determine whether there are correlates of thyroid dysfunction with disease progression. METHODS Total and free thyroxine, triiodothyronine, reverse triiodothyronine, thyrotropin, and thyroxine binding globulin (TBG) were measured in 167 children with HIV infection (age, 1 to 19 years; mean, 9.15 years). METHODS Pediatric Branch, National Cancer Institute. RESULTS Free thyroxine was at or below the lower limit of normal (normal, 1.0 to 1.9 ng/dl) in 18% of the children; thyrotropin and TBG levels were above the normal range in 31% and 30%, respectively. There was an inverse correlation between CD4+ cell count and thyrotropin, and between CD4+ cell count and TBG. No correlation was found between thyroid function and other disease symptoms or medications. CONCLUSIONS These findings indicate that thyroid abnormalities occur more frequently in children with HIV infection than was previously reported, have a different profile from the thyroid abnormalities associated with other chronic disease conditions, and correlate with disease progression.