BACKGROUND: Roux-en-Y gastric bypass (RYGB) modifies the anatomical structure of 
the upper intestine tract, reduces gastric acid secretion, and may impair LT4 
absorption. The aim of this study was to evaluate the LT4 absorption in morbidly 
obese patients before and after RYGB.
METHODS: Thirty morbidly obese patients were divided in two groups: The NS group 
included 15 patients before RYGB surgery (BMI = 43.1 ± 4 kg/m(2)), and the S 
group included 15 patients after surgery (BMI = 37.3 ± 4 kg/m(2)). Two baseline 
samples were collected, and 600 μg of oral LT4 tablets were administered. Blood 
samples were collected at 30, 60, 120, 180, 240, 300, and 1440 min. Serum-free 
T4 (FT4), total T4 (TT4), and TSH were measured at each time point. The increase 
in TT4, FT4, and TSH (ΔTT4, ΔFT4, and ΔTSH) was calculated, subtracting from the 
baseline mean value.
RESULTS: The pharmacokinetics parameters regarding LT4 absorption, maximum ΔTT4, 
and area under the curve(AUC) of both ΔTT4 and ΔFT4 were significantly higher in 
the S group compared with the NS group (p < 0.05). It was observed, however, 
that there was a significant delay in the absorption of LT4 in the S group. 
Basal serum TSH and leptin levels were higher in the NS group (p = 0.016 and 
0.026, respectively), whereas basal serum TT4, FT4, ΔTSH, and the AUC of ΔTSH 
were similar between groups.
CONCLUSIONS: In this study, we have demonstrated that Roux-en-Y bypass surgery 
does not diminish LT4 absorption. A small but significant delayed absorption of 
LT4, however, was observed in patients after surgery.