Neuropsychological Function and Symptoms in Subjects with Subclinical Hypothyroidism and the Effect of Thyroxine Treatment

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Abstract
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 T4 and free T3 levels, and to study the effect of T4 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 Tromsø study). Sixty-nine of those with SHT were included in a placebo-controlled, double-blind intervention study with T4 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 T4 group (n = 36) and the placebo group (n = 33) were 1.52 ± 1.51 and 5.42 ± 1.96 mIU/liter, respectively. T4 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.

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