Frank hypothyroidism is known to induce neurological and mental dysfunction. The aim of this study was to assess selected neuropsychological and behavioral features by means of standardized tests in a group of 14 patients with subclinical hypothyroidism who were free from neuropsychological complaints and to evaluate the possible effects of L-thyroxine treatment on their performance. Patients were submitted to the Crown and Crisp Experiential Index and to the Wechsler Memory Scale; their ratings on the neurobehavioral tests and their thyroid hormone profile were compared to those of a control group of 50 age- and sex-matched subjects. Comparison was also carried out between pretreatment ratings and those obtained following a 6-month L-thyroxine course (0.1-0.15 mg/day). The Wechsler Memory Scale ratings showed a significant impairment in patients' memory-related abilities [memory quotient (MQ) = 89.1 +/- 2.9; P = 0.002 (patients versus controls)]; the Crown and Crisp Experiential Index ratings demonstrated moderate differences between untreated patients and controls with respect to hysteria (P = 0.03), anxiety (P = 0.05), somatic complaints (P = 0.0005), and depressive features (P = 0.002) scales; the total score was also significantly higher (42.0 +/- 3.8; P = 0.005). After L-thyroxine treatment the patients' performances showed an improvement in memory skills, as evaluated by the Wechsler Memory Scale [MQ = 99.9 +/- 4.0; P = 0.002 (treated versus untreated)]; somatic complaints (P = 0.02) and obsessionality (P = 0.04) ratings and the Crown and Crisp Experiential Index total score (P = 0.04) significantly decreased with respect to untreated patients.(ABSTRACT TRUNCATED AT 250 WORDS)