After bariatric surgery (BS)-induced weight loss in people with hypothyroidism, a retrospective, observational study suggested that levothyroxine (LT4) dosing using the patient’s ideal body weight was the best method to use for medication adjustment. Researchers who published this finding in the Journal of Clinical Medicine noted that there are no clear data regarding what happens to the dose of LT4 after BS, which alters the gastrointestinal tract and may change absorption of the drug.

Patients (N=48) who were morbidly obese (body mass index [BMI] >40 kg/m2 or >35 kg/m2 with  ³1 serious  obesity-related health problem), had hypothyroidism, and underwent BS at the University Hospital of A Coruña in Spain between 2016 and 2020 were assessed for LT4 dosing up to 12 months after surgery.

The study population comprised mostly women (n=45), aged median 47.2 (interquartile range [IQR], 39.2-52.3) years, with a median BMI of 47.1 (IQR, 42.5-49.4) kg/m2. Fifty-six percent (56%) underwent Roux-en-y gastric bypass, and 44% underwent sleeve gastrectomy.


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Compared with baseline, absolute LT4 dose and LT4 dose/kg ideal body weight (both P = .058) did not differ significantly 12 months after surgery. However, the LT4 dose/ body surface area the patients were receiving was on average 30.3% higher than before surgery. This average increase was 69.8% for LT4 dose/kg actual weight, 33.4% for LT4 dose/kg adjusted body weight, 178.7% for LT4 dose/kg body fat, and 18.9% for LT4 dose/kg lean body weight.

After adjusting for time and surgical procedure, LT4 dose/body surface area (b, 0.88; P <.001), LT4 dose/kg actual weight (b, 0.03; P <.001), LT4 dose/kg adjusted body weight (b, 0.03; P <.001), LT4 dose/kg body fat (b, 0.18; P <.001), and LT4 dose/kg lean body weight (b, 0.02; P <.001) differed significantly. Neither LT4 dose (b, 0.65; P =.142) nor LT4 dose/kg ideal body weight (b, 0.01; P =.095) changed significantly over time.

The researchers acknowledged that the study has several limitations, including a small sample size, which may have influenced whether there were statistically significant differences between sleeve gastrectomy and Roux-en-Y gastric bypass patients.

“The present study shows that in hypothyroid patients with morbid obesity, after BS-induced weight loss, the total LT4 dose or the LT4 dose/kg ideal body weight did not change, although there was a significant increase in LT4 dose/[body surface area], LT4 dose/kg actual weight, LT4 dose/kg adjusted body weight, LT4 dose/kg body fat, and LT4 dose/kg lean body weight,” according to the researchers, adding that the study “strongly suggests that thyroid-stimulating hormone in obese hypothyroid patients can be individually adapted more precisely if it is based on ideal body weight.”

Reference

Juiz-Valiña P, Cordido M, Outeiriño-Blanco E, et al. Evaluation of thyroid hormone replacement dosing in morbidly obese hypothyroid patients after bariatric surgery-induced weight loss.  J Clin Med. 2021;10(16):3685. doi:10.3390/jcm10163685