Levothyroxine dose requirements, oral bioavailability and gut microbiome enterotype dynamics during the early post-bariatric period: a single-centre, prospective, cohort study.
El-Sehrawy A, Gameil M, et al. • BMC endocrine disorders • 2026
Bariatric surgery was associated with substantial metabolic improvement, increased LT4 requirements, increased systemic FT4 exposure, and changes in gut microbiome composition, highlighting the need for individualized thyroid function monitoring and LT4 dose adjustment during the postoperative period.
Key Findings
Results
Both RYGB and SG produced significant reductions in BMI and fasting blood sugar at 6 months postoperatively.
BMI decreased from 44.5 to 32.1 kg/m² in RYGB and from 42.9 to 33.0 kg/m² in SG (both p < 0.001).
Fasting blood sugar decreased from 114.8 to 92.4 mg/dL in RYGB and from 109.9 to 96.1 mg/dL in SG (both p < 0.001).
79 patients with hypothyroidism were followed at baseline, 3 months, and 6 months postoperatively.
RYGB group: n = 40; SG group: n = 39.
Linear mixed-effects models were used to assess longitudinal changes.
Results
Levothyroxine dose requirements increased significantly after bariatric surgery, with a stronger effect in RYGB than SG.
LT4 dose increased from 137.5 to 158.2 µg/day in the RYGB group (p = 0.018).
LT4 dose increased from 133.3 to 144.8 µg/day in the SG group (p = 0.067 for absolute dose).
Weight-adjusted LT4 dose increased significantly in both groups (both p < 0.05).
After false discovery rate (FDR) adjustment, LT4 dose (µg/kg/day) remained significant particularly in the RYGB group.
These changes were observed across follow-up at 3 and 6 months postoperatively.
Results
FT4 oral bioavailability, measured by AUC₀₋₆h, increased significantly after both types of bariatric surgery.
FT4 AUC₀₋₆h increased by +330 ± 210 in the RYGB group (p = 0.003).
FT4 AUC₀₋₆h increased by +185 ± 190 in the SG group (p = 0.041).
These increases indicate altered LT4 pharmacokinetics following surgery.
The greater increase in AUC in RYGB compared to SG suggests procedure-specific differences in drug absorption.
Results
Gut microbiome Shannon diversity increased after RYGB surgery.
Shannon diversity showed a significant positive trend in the RYGB group (p-trend = 0.048).
Gut microbiome diversity was assessed using diversity metrics alongside microbiome composition analysis.
The study examined gut microbiome enterotype dynamics longitudinally at baseline, 3 months, and 6 months.
The increase in diversity was more pronounced in the RYGB group compared to SG.
Results
Akkermansia muciniphila abundance increased significantly after bariatric surgery, particularly in the RYGB group.
A. muciniphila abundance increased from 1.20% to 2.40% (p < 0.001) following surgery.
This enrichment was described as particularly notable in the RYGB group.
After FDR adjustment, A. muciniphila changes remained significant in the RYGB group.
The enrichment of A. muciniphila represents one of the major longitudinal microbiome changes identified.
Results
Changes in Akkermansia muciniphila abundance showed a modest positive association with FT4 AUC₀₋₆h after multivariable adjustment, but other microbiome-LT4 associations were not significant.
The association between A. muciniphila abundance changes and FT4 AUC₀₋₆h was described as 'modest' and detected in patient-level analyses after multivariable adjustment.
Other microbiome-LT4 pharmacokinetic associations were not statistically significant.
The authors characterized this finding as 'exploratory and hypothesis-generating rather than causal.'
The analysis was conducted using patient-level data with multivariable adjustment.
Methods
The study design was a single-centre, prospective cohort following hypothyroid patients undergoing bariatric surgery over 6 months.
79 patients with hypothyroidism undergoing bariatric surgery were enrolled (RYGB n = 40; SG n = 39).
Follow-up assessments occurred at baseline, 3 months, and 6 months postoperatively.
Outcomes included metabolic parameters, thyroid function tests, LT4 dose, FT4 AUC₀₋₆h, and gut microbiome diversity.
Statistical analyses employed linear mixed-effects models and false discovery rate adjustment for multiple comparisons.
The study was conducted at a single centre.
What This Means
This research suggests that people with an underactive thyroid (hypothyroidism) who undergo weight-loss surgery may need adjustments to their thyroid medication (levothyroxine) in the months following the procedure. In a study of 79 hypothyroid patients followed for six months after either Roux-en-Y gastric bypass (RYGB) or sleeve gastrectomy (SG), both types of surgery led to significant improvements in metabolic health, including large reductions in BMI and blood sugar. However, the amount of thyroid medication patients needed increased — especially after RYGB — and the amount of the drug actually absorbed into the bloodstream also rose substantially, suggesting that surgery alters how the body processes levothyroxine.
The study also found that the gut microbiome — the community of bacteria living in the digestive tract — changed meaningfully after surgery. Bacterial diversity increased, particularly after RYGB, and a specific bacterium called Akkermansia muciniphila became more abundant, roughly doubling in proportion in both surgery types. There was a modest statistical association between the rise in A. muciniphila and increased levothyroxine absorption, though the researchers caution this finding is preliminary and does not prove the bacteria cause the change in drug absorption.
This research suggests that patients with hypothyroidism who have bariatric surgery should have their thyroid hormone levels monitored closely after the operation, as their levothyroxine dose may need to be recalibrated. The findings also open questions about the role the gut microbiome may play in how thyroid medications are absorbed, which could be relevant for understanding individual differences in medication response more broadly.
El-Sehrawy A, Gameil M, Nakib A, Aseeri A, Alanazi M, Alharbi S, et al.. (2026). Levothyroxine dose requirements, oral bioavailability and gut microbiome enterotype dynamics during the early post-bariatric period: a single-centre, prospective, cohort study.. BMC endocrine disorders. https://doi.org/10.1186/s12902-026-02599-y