Does One Size Fit Everyone? Replacement Dose of Levothyroxine in Long-standing Primary Hypothyroidism in Adults

OBJECTIVES The recommended starting dose of levothyroxine (LT4) in primary hypothyroidism is 1.6 μg/kg body weight and is based on presumption of minimal residual thyroid function in autoimmune hypothyroidism. This study aimed at finding the range and determining factors for LT4 dose in long-standing hypothyroidism. METHODS A cross-sectional study of individuals with primary autoimmune hypothyroidism on LT4 replacement was done between March 2015 and January 2016. Individuals enrolled were euthyroid based on recent serum thyroid-stimulating hormone. The inclusion criteria included LT4 intake in the morning empty stomach, maintenance of at least 1-h food gap, not on medications known to hamper LT4 absorption within 4 h of dosing, diagnosis of hypothyroidism at least for 1 year, and on a minimum 25 μg LT4. P < 0.05 was considered statistically significant. RESULTS A total of 346 individuals (290 women and 56 men; 214 premenopausal and 76 postmenopausal women) were enrolled. The mean duration of hypothyroidism and age were 5.7 years and 42.1 years, respectively. The range and mean of absolute LT4 daily dose (ADD), LT4 dose based on body weight (D/W), and LT4 dose based on ideal body weight (D/IBW) were 25-200 μg daily and 77.1 μg, 0.3-2.82 μg/kg and 1.21 μg/kg, and 0.42-3.5 μg/kg and 1.58 μg/kg, respectively. Duration of hypothyroidism was significant predictors of ADD, D/W, and D/IBW. Gender-based difference in ADD and D/IBW was explained by gender difference in anthropometry. CONCLUSION Long-standing primary autoimmune hypothyroidism has variable dose requirement of LT4 for achieving euthyroidism and may be dependent on the degree of residual functional thyroid. Duration of hypothyroidism was significant positive predictor for either ADD, D/W, or D/IBW.