Objective: A great number of factors can interfere with levothyroxine (LT4) tablet absorption, leading to increased serum thyroid-stimulating hormone (TSH) levels and, accordingly, to increased LT4 requirements. LT4 oral solution (LT4-OS) is a novel formulation with a pharmacokinetics profile different from those of tablets. The aim of this study was to retrospectively evaluate whether serum TSH levels were decreased after switching adult hypothyroid patients from the tablet to LT-OS.
Methods: We retrospectively evaluated 53 outpatients on LT4 replacement therapy (consumed within 1 hour before breakfast) who switched from LT4 tablets to LT4-OS without changing the daily dose. We compared preswitch TSH (TSH1) with TSH level 60 to 90 days after the switch (TSH2) and examined the clinical differences between the patients whose TSH did and did not drop after the switch.
Results: After the switch, TSH levels decreased from a median value of 3.04 (interquartile range [IQR] 1.75-6.80) to 2.30 (IQR 1.21-3.81) μIU/mL, and the difference was significant (P = .0034). We observed a TSH reduction (TSH2/TSH1 ratio <1) in 36/53 (67.9%) of patients; the median TSH2/TSH1 ratio was 0.71 (IQR 0.37-1.14). In the group of patients whose TSH dropped, we observed an increased frequency of factors interfering with LT4 absorption (P = .014). The median TSH2/TSH1 ratios were 0.50 (IQR 0.31-0.72) and 0.85 (IQR 0.65-1.36) for patients with and without interfering factors, respectively.
Conclusion: Our study confirms that LT4-OS could have an increased absorption rate in comparison to LT4 tablets, especially in the presence of other factors interfering with LT4 absorption.