Low-Normal or High-Normal Thyrotropin Target Levels During Treatment of Hypothyroidism: A Prospective, Comparative Study

Abstract

Recent literature advocates the decrease of the upper limit of the normal thyrotropin (TSH) reference range. The objective of this study was to determine whether treated hypothyroid patients maintained within a low-normal TSH range (0.4-2.0 mIU/L) have better clinical outcomes than those maintained within a high-normal TSH range (2.0-4.0 mIU/L). The study was performed in a thyroid outpatient clinic of a tertiary hospital. This was a prospective, interventional study. Forty-two participants with newly diagnosed overt primary hypothyroidism were paired in two groups: group 1 (n=20), low-normal target TSH; group 2 (n = 22), high-normal target TSH. Levothyroxine was initiated, and dose was adjusted to achieve and sustain the target TSH value during the study period. After the target TSH was reached, participants were evaluated every 3 months for thyroid function, serum lipid profile, resting energy expenditure (REE), body composition, and bone mineral density, for 12 months. Nineteen patients in group 1 and 16 in group 2 completed the study. In the whole-group analysis, total cholesterol (p = 0.01), low-density lipoprotein cholesterol (p = 0.004), and triglycerides (p < 0.001) decreased after treatment, whereas REE per kilogram of lean body mass (p = 0.001) and total fat body mass (p =0.02) increased. Group 1 patients had a significantly higher relative increase in REE (+7.1% ± 11.3% vs. +3.6% ± 15.1%, p = 0.02). There was no difference between the groups in the other variables. Despite recent trends toward lowering the upper limit of normal TSH range, the results of this 12-month study provided no substantial clinical evidence to corroborate that treatment of primary hypothyroidism should aim at maintaining TSH levels in a low-normal range