Levothyroxine Replacement In Obese Hypothyroid Females After Total Thyroidectomy

Abstract

Levothyroxine (LT4) replacement in hypothyroid obese patients is poorly understood. We assessed whether the LT4 regimen required to achieve euthyroidism differs between nonobese and obese hypothyroid females. We retrospectively identified nonobese and obese females who received LT4 starting with a standard dose of 1.6 μg/kg after total thyroidectomy for preoperative diagnosis of benign goiter. We examined the association between LT4 dosage required to achieve euthyroid state (thyroid-stimulating hormone [TSH] 0.4-2.5 mIU/L) and patient characteristics using linear regression models with and without adjustment for age, ethnicity, medication use, and postoperative hypoparathyroidism. We identified 32 females (15 nonobese/17 obese) who achieved euthyroid state. Obese patients weighed more (104.1 ± 22.5 vs. 64.9 ± 10.0 kg, P<.0001) and required a higher final LT4 than nonobese (146 ± 38 vs. 102 ± 12 μg, P = .0002) but LT4 requirements per kg total body weight (TBW) were similar (1.60 ± 0.29 vs. 1.42 ± 0.38 μg/kg, P = .15). LT4 dose per kg ideal body weight (IBW) was higher in obese than in nonobese females (2.62 ± 0.67 vs. 1.88 ± 0.28 μg/kg, P = .0004) and this difference persisted after adjustments (P<.05). During LT4 titration, 47% and 20% of obese and nonobese patients had subnormal TSH episodes, respectively (P = .11). After taking LT4 compliance, malabsorption, and competing medication use into consideration, we found marked LT4 dose variability in obese patients. Patients who needed a mean daily LT4 dose ≤150 mg (124 ± 16 μg/day) compared with >150 μg (198 ± 4 μg/day) demonstrated lower LT4 per TBW (1.25 ± 0.18 vs. 1.84 ± 0.43 μg/kg, P = .03) and IBW (2.28 ± 0.47 vs. 3.44 ± 0.18 μg/kg, P<.0001), respectively. The standard approach to LT4 replacement in obese and nonobese females after thyroidectomy is imprecise. Mean daily LT4 doses in obese and nonobese patients were similar if expressed per kg TBW, though there was variability in the final LT4 among obese patients. We suggest initiating LT4 at a dose lower than that routinely recommended in obese females.