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Submitted on August 8, 2005
Accepted on October 25, 2005
Institute of Clinical Medicine, University of Tromsø, Tromsø, Norway (R.J.), Department of Internal Medicine (H.S., A.N.) and Department of Clinical Chemistry (J.S.), University Hospital of North Norway, Tromsø, Norway, Department of Nephrology, National Hospital, Oslo, Norway (T.G.J.)
* To whom correspondence should be addressed. E-mail: rolf.jorde{at}unn.no.
Objective: To examine the relation between neuropsychological function and subclinical hypothyroidism (SHT), defined as serum TSH (TSH) 3.5 - 10.0 mIU/L and normal serum free thyroxine and free triiodothyronine levels, and to study the effect of thyroxine supplementation.
Subjects: 89 subjects (45 males) with SHT and 154 control subjects (72 males) recruited from a general health survey (the 5th Tromsø study). 69 of those with SHT were included in a placebo-controlled, double-blind intervention study with thyroxine medication for one year.
Main outcome measures: 14 tests of cognitive function, Beck Depression Inventory, General Health Questionnaire (GHQ-30), 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/L, 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 thyroxine group (n = 36) and the placebo group (n = 33) were 1.52 ± 1.51 and 5.42 ± 1.96 mIU/L, respectively. Thyroxine 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/L range there is no neuropsychological dysfunction, and compared with healthy controls, there is no difference in symptoms related to hypothyroidism.
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