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

Institute of Clinical Medicine (R.J., K.W.), University of Tromsø, 9037 Tromsø, Norway; Departments of Internal Medicine (H.S., A.N.) and Clinical Chemistry (J.S.), University Hospital of North Norway, 9038 Tromsø, Norway; and Department of Nephrology (T.G.J.), National Hospital, 0027 Oslo, Norway

Address all correspondence and requests for reprints to: Rolf Jorde, Medical Department B, University Hospital of North Norway, 9038 Tromsø, Norway. E-mail: rolf.jorde{at}unn.no.

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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