Neuropsychological function and symptoms in subjects with subclinical hypothyroidism and the effect of thyroxine treatment

Objective: Our objective was to examine the relation between neuropsychological function and subclinical hypothyroidism (SHT), defined as serum TSH of 3.5-10.0 mIU/liter and normal serum free T-4 and free T-3 levels, and to study the effect of T-4 supplementation. Subjects: A total of 89 subjects (45 males) with SHT and 154 control subjects (72 males) were recruited from a general health survey (the fifth Tromso study). Sixty-nine of those with SHT were included in a placebo-controlled, double-blind intervention study with T-4 medication for 1 yr. Main Outcome Measures: We used fourteen tests of cognitive function, Beck Depression Inventory, General Health Questionnaire, and a questionnaire on hypothyroid symptoms. Results: The mean +/- SD serum TSH in the SHT and control group were 5.57 +/- 1.68 and 1.79 +/- 0.69 mIU/liter, respectively. There were no significant differences in cognitive function and hypothyroid symptoms between the two groups, but those with SHT scored significantly better than the controls on the GHQ-30. At the end of the intervention study, serum TSH in the T-4 group (n=36) and the placebo group (n=33) were 1.52 +/- 1.51 and 5.42 +/- 1.96 mIU/liter, respectively. T-4 substitution had no effect on any of the parameters measured. Conclusion: In subjects with SHT where the serum TSH level is in the 3.5-10.0 mIU/liter range, there is no neuropsychological dysfunction, and compared with healthy controls, there is no difference in symptoms related to hypothyroidism.