Improved Levothyroxine Pharmacokinetics After Bariatric Surgery

Background: The absorption of levothyroxine (LT4) is affected by many factors. Bariatric surgery is recommended in severely obese patients. The aim of this study was to determine the consequences of bariatric surgery on LT4 pharmacokinetic parameters, and to identify the regions of the gastrointestinal tract where LT4 is absorbed in patients with severe obesity before and after surgery. Methods: We studied 32 severely obese nonhypothyroid patients who underwent sleeve gastrectomy (SG; n =10), Roux-en-Y gastric bypass (RYGBP; n =7), or biliopancreatic diversion with long limbs (BPD-LL; n =15). Before surgery, from 8:00 a.m., blood samples were collected before and every 30 minutes after the oral administration of a solution of 600 μg of LT4. The same procedure was repeated 35 days after surgery. We estimated the pharmacokinetic parameters of LT4 before and after surgery, including the area under the curve (AUC), the peak thyroxine concentration ( C max ), and the time to peak thyroxine concentration ( T max ). Results: Following surgery, in the SG group, the mean AUC was higher than it was before surgery (18.97±6.01 vs. 25.048±6.47 [μg/dL]·h; p