Levothyroxine expired
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Expired Levothyroxine: Stability, Bioavailability, and Alternative Uses
Stability of Expired Levothyroxine Tablets
Levothyroxine, a widely prescribed medication for thyroid hormone replacement, has faced numerous stability issues over the years. Research indicates that the stability of levothyroxine sodium pentahydrate tablets is significantly influenced by the excipients used in their formulation. Tablets containing lactose anhydrous, starch, or microcrystalline cellulose tend to degrade and fail to meet potency standards within three months when stored at 40°C and 75% relative humidity. Conversely, tablets formulated with dibasic calcium phosphate or mannitol maintain their potency for up to three months under the same conditions, but not beyond six months. The inclusion of basic pH modifiers such as sodium carbonate, sodium bicarbonate, or magnesium oxide can enhance the stability, allowing the tablets to meet potency requirements for up to six months.
Bioavailability Concerns with Levothyroxine
The bioavailability of levothyroxine can be compromised by various medications and foods. A systematic review has identified several substances that interfere with levothyroxine absorption, including calcium and iron supplements, proton pump inhibitors, bile acid sequestrants, phosphate binders, sex hormones, and anticonvulsants. Certain foods and beverages can also induce malabsorption. The mechanisms behind these interactions include direct complexing, alkalization, alteration of serum thyroxine-binding globulin levels, and acceleration of levothyroxine catabolism via deiodination. To mitigate these interactions, dose adjustments, administration separation, and discontinuation of interfering substances are recommended. Additionally, liquid solutions and soft-gel capsules of levothyroxine can help eliminate malabsorption issues caused by chelation and alkalization.
Alternative Use of Expired Levothyroxine as a Corrosion Inhibitor
Interestingly, expired levothyroxine has been found to have an alternative use as a corrosion inhibitor for stainless steel 304L in acidic environments. Studies have shown that expired levothyroxine can effectively inhibit the corrosion of stainless steel in a 0.5 M H2SO4 solution. The inhibition efficiency increases with higher doses of the expired drug but decreases with rising temperatures. The corrosion inhibition is primarily due to the adsorption of the drug onto the steel surface, following the Langmuir adsorption isotherm. Techniques such as mass loss measurements, potentiodynamic polarization tests, and electrochemical impedance spectroscopy have confirmed these findings. The surface morphology analysis using AFM, FTIR, SEM, and EDX further supports the protective effect of expired levothyroxine on stainless steel.
Conclusion
Expired levothyroxine, while potentially less effective for its intended medical use due to stability issues, can still serve valuable purposes. Its stability can be influenced by the choice of excipients and storage conditions, and its bioavailability can be affected by various interactions with other substances. Moreover, expired levothyroxine has shown promise as an eco-friendly corrosion inhibitor for stainless steel, highlighting a novel application for this otherwise discarded medication.
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