Findings on thyroid function in blind subjects are lacking. The aim of this study was to investigate the thyroid hormonal pattern in prepubertal blind subjects. Six healthy and 8 blind males, aged 7-10 yr, in Tanner stage one puberty, living at Institute "Martuscelli" for blind young subjects, Napoli, Italy, were studied. Each had a TRH (200 micrograms) test at 08:00 h after nocturnal rest. Plasma TSH, T4, T3, free T4(FT4), free T3(FT3) and cortisol (F) were measured by RIA. Our blind subjects show levels of TSH (basal values and absolute peak after TRH), T4, T3 and F normal but FT4 levels significantly higher than controls (39 pg/ml +/- 4.7 vs 12 +/- 0.6, p less than 0.001; 14 pg/ml +/- 1.3 vs 4.7 +/- 0.2, p less than 0.001, respectively). Our results, similar to those found in some patients with euthyroid hyperthyroxinemia, suggest that the prolonged inability to receive light signal could influence the metabolism of thyroid hormones and/or cause a tissue resistance to their action, even if this hypothesis must be verified by future more extensive investigations.