Optimal daily levothyroxine dose in primary hypothyroidism. Its relation to pretreatment thyroid hormone indexes.

Several studies have described the mean optimal replacement daily dose of levothyroxine sodium in subjects with primary hypothyroidism. Furthermore, a cautious approach in replacement therapy of the elderly population is well established. However, the initial dose of levothyroxine has remained variable, especially in young subjects, since the guidelines for the prediction of the final replacement dose are scarce. This study assessed the efficacy of pretreatment thyroid hormone indexes in determination of the final maintenance levothyroxine dose in 156 subjects with primary hypothyroidism. The optimal daily levothyroxine dose as defined by normalization of serum thyroxine and basal thyroid-stimulating hormone levels, as well as thyroid-stimulating hormone response to the intravenous administration of thyrotropin-releasing hormone, varied between 50 micrograms and 200 micrograms and, hence, the subjects were grouped according to the final levothyroxine dose. Significant correlations were noted between the levothyroxine dose for individual groups and pretreatment serum triiodothyronine, thyroxine, and thyroid-stimulating hormone concentrations. However, the serum thyroid-stimulating hormone concentration demonstrated a markedly closer relationship with the levothyroxine dose than did either triiodothyronine or thyroxine for both comparisons. Furthermore, when assessed for all subjects, the correlation between levothyroxine dose and thyroid-stimulating hormone level, alone, remained significant. This study demonstrates that obtaining routine thyroid hormone indexes prior to initiation of replacement levothyroxine therapy may be reliable predictors of the final levothyroxine dose; serum thyroid-stimulating hormone concentration being the best index. Thus, the levothyroxine dose may be reliably predicted depending on a specific range of thyroid-stimulating hormone levels. Therefore, in an individual subject, aiming for the predicted dose rather than the mean dose, a procedure described in the medical literature, may be more helpful in arriving at an optimal levothyroxine dose.