Introduction: The anatomical and physiological changes induced by bariatric surgery (BS) can significantly impact the absorption and pharmacokinetics of orally administered medication. In particular, levothyroxine (LT4) dosage, given its dependence on patients weight and gastric pH for optimal absorption, is expected to change following BS. However, the literature is not unanimous and there is a void of longer-term studies. We aimed to compare LT4 dosage 4 years following BS, comparing malabsorptive versus restrictive techniques.
Methods: This was a retrospective observational study performed in a single center between 2010 and 2017 that included patients that underwent BS (Roux-en-Y gastric bypass (RYGB) and gastric sleeve (GS) and completed a 4-year follow up. All hypothyroid patients under LT4 (n=86, 92% female, age: 48±11 years, BMI: 43.1±4.3 kg/m2) were selected from a total of 1070 patients (8%). GS was performed in 39 (45.3%) and RYGB in 47 (54.7%). 44 patients had autoimmune thyroiditis (AIT), 33 had thyroid resection, 3 had central hypothyroidism, 6 unclarified etiology.
Results: Four years after surgery, 23 (59%) patients that underwent GS and 22 (46.8%) that underwent RYGB required a lower daily dose of LT4, had to be increased in 6 (15.4%) and 10 (21.3%) and remained unchanged in 10 (25.6%) and 15 (31.9%), respectively. No statistically significant differences were found between types of surgery. For both GS and RYGB we observed statistically reduced values of total daily dose of LT4 at years 1 and 4 when compared to baseline (P<0.001), while the average weight-based daily LT4 was significantly increased at years 1 and 4 when compared to baseline (1.31 μg/kg (±0.55) at baseline, 1.48 μg/kg (±0.51) at year 1 and 1.42 μg/kg (±0.55) at year 4 for GS; 1.11 μg/kg (±0.44) at baseline, 1.47 μg/kg (±0.55) at year 1 and 1.41 μg/kg (±0.47), P<0.001). In pairwise comparisons, doses at year 1 and 4 are not statistically different. The same pattern was found independently of hypothyroidism etiology.
Conclusion: Our results are consistent with previous work, demonstrating that following BS, individuals with hypothyroidism and obesity frequently require a lower dose of LT4 per body weight. We further demonstrate that this trend is sustained for at least 4 years. Nevertheless, there is variability, with some patients requiring dosage increment or no adjustment, highlighting the importance of tight and individualized monitorization of patients with hypothyroidism after BS.
13 May 2023 - 16 May 2023