Efficacy of a levothyroxine dosage regimen based on serum thyrotropin level, for primary hypothyroidism. An open label dose finding study.

BACKGROUND: There are no guidelines on individualized initial levothyroxine dosage in primary hypothyroidism. This prospective observational study was done to assess whether a predetermined dose of levothyroxine based on Thyroid Stimulating Hormone (TSH) levels would be able to make the patient euthyroid during a period of six weeks and to find other factors which influence the levothyroxine requirement.

MATERIALS AND METHODS: Newly diagnosed patients with primary hypothyroidism or those patients who were not on levothyroxine therapy were divided into TSH-based groups-Group 1, 5-9.99, Group 2, 10-29.99, Group 3, 30-99.99 and Group 4, >100 μIU/ml and treated with an initial levothyroxine dose of 25,50,75 and100 μg/day for next six weeks. Factors correlating with levothyroxine requirement were determined.

RESULTS: Of the 171 patients who were included 142 completed the study, 34,46,28 and 34 patients were included in groups 1 to 4, respectively. Normalization of TSH with the above criteria was achieved in 111 (78.7%) out of 141 patients, and 91%, 67%, 75%, and 82% respectively in the 4 groups. Among adequately replaced patients pre-treatment TSH level (r = 0.81), T4 level (r = 0.61), and body weight (r = 0.19) correlated with the levothyroxine requirement. Based on these factors predicted initial dose (μg/day) was found to be 0.54 (Body Weight [Kg]) +0.47 (TSH [μIU/m]) - 1.4 (Total T4 [μg/dl]) +17.79 or 0.27 (Body Weight) +0.553 (TSH) +21.

CONCLUSION: Serum thyrotropin-based categorization for initial levothyroxine dose leads to euthyroidism in nearly four of five patients with primary hypothyroidism. The dose required for adequate replacement of levothyroxine has correlation with pre-treatment serum TSH levels serum thyroxine levels and body weight.