Objective: To determine factors that influence clinicians' decisions to treat subclinical hypothyroidism. Patients and Methods: We studied patients living within 120 miles of our institution in Rochester, Minn, whose serum thyrotropin value was between 5.1 and 10.0 mIU/L from January 1, 1995, through December 31, 1996. Exclusion criteria were history of thyroid disease or prior abnormal thyrotropin level and fewer than 5 follow-up visits or fewer than 2 follow-up thyrotropin measurements. Of the 2655 patients in this cohort, only 55% (1466) had measurement of thyroid microsomal antibodies (TMAb). From this group, records of 539 patients were selected randomly according to antibody status (238 with positive, 199 with negative, and 102 with no TMAb measurement). After exclusion of patients receiving thyroxine replacement and other medications affecting thyroid function, patients with postpartum thyroiditis, and patients in the intensive care unit, data from 450 patients were analyzed. Results: Thyroxine therapy was prescribed for 39% of the patients with thyrotropin levels between 5.1 and 10.0 mIU/L. In multivariate analysis, TMAb status (P<.001), index thyrotropin (P<.002), and free thyroxine (FT4) (P=.03) were the strongest predictors for the decision to treat. Fifty-nine percent of the patients with positive TMAb were treated compared with 24% of those with negative TMAb and 30% of those who did not have a TMAb test. The mean thyrotropin level in the treated group was 7.1 mIU/L compared with 6.3 mIU/L in the untreated group. The mean FT4 level was 1.08 ng/dL in the treated group who had this measured compared with 1.16 ng/dL in the untreated group. Treated patients were younger than untreated patients (56.5 years vs 60.6 years), and the proportion of female patients was higher in the treated group compared with the untreated group (75.8% vs 65.2%); however, neither of these differences was significant after correction for the multiple comparisons. In multivariate analysis neither age nor sex was a significant predictor of treatment, but in the subgroup comparison, patients aged 31 to 50 years were twice as likely to be treated compared with those aged 61 to 80 years. Conclusion: Among patients with serum thyrotropin levels of 5.1 to 10.0 mIU/L, those with positive TMAb values, those with higher thyrotropin values, and those with lower FT4 values were more likely to receive thyroxine therapy. Younger patients were more likely to be treated than older patients.
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