SERUM THYROTROPIN IN PRIMARY HYPOTHYROIDISM - A POSSIBLE PREDICTOR OFOPTIMAL DAILY LEVOTHYROXINE DOSE IN PRIMARY HYPOTHYROIDISM

Background: Pretreatment thyrotropin levels may be a reliable predicto r of the optimal daily dose of levothyroxine sodium in patients with p rimary hypothyroidism. However, the older method of serum thyrotropin determination, with the reference range of less than 1 to 8 mU/L, has given way to a newer, supersensitive thyrotropin assay, with a referen ce range of 0.5 to 5.0 mU/L. Thus, at present, the previously establis hed relationship between the levothyroxine dose and the pretreatment s erum thyrotropin concentration may not be reliable in predicting the o ptimal daily dose of levothyroxine. Methods: We reassessed the relatio nship between the optimal daily levothyroxine dose and the pretreatmen t serum thyrotropin concentration as determined by the newer assay in 192 consecutive patients with primary hypothyroidism referred to an en docrinology clinic over a period of 4 years. Results: The optimal dail y dose of levothyroxine sodium ranged from 25 to 225 mu g, with most p atients (65%) requiring 100 to 150 mu g/d and a median dose of 125 mu g. Multiple regression analysis documented a significant curvilinear c orrelation between the mean pretreatment serum thryrotropin concentrat ion and the optimal daily levothyroxine dose for individual groups div ided according to available tablet strengths (r=.994, P<<.001). A simp le linear regression was also significant (r=.92, P<.001), although wi th an intercept much higher than the minimum levothyroxine sodium dose of 25 mu g/d. However, the relationships markedly improved when the l inear regressions were determined separately for two further subgroups at the median daily dose of 125 IJ 8, providing equations to predict even the smallest optimal daily dose of levothyroxine. Conclusion: Pre treatment thyrotropin levels determined by new assays may also provide a useful guideline in determining the optimal daily maintenance dose of levothyroxine in patients with primary hypothyroidism.