Drug interactions
Levothyroxine + Metformin
Severity: Moderate
Levothyroxine and Metformin: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Correcting hypothyroidism or over-replacing with levothyroxine can raise blood glucose and change glycemic control in patients on metformin. Metformin may also lower TSH in some patients with hypothyroidism.
What this interaction means in plain words
Levothyroxine and Metformin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Levothyroxine belongs to Endocrine; Metformin belongs to Endocrine.
Why it happens
Thyroid hormone increases hepatic glucose output and glucose turnover, opposing the glucose-lowering effect of metformin.
What to do
Monitor blood glucose and HbA1c when levothyroxine is started or its dose changed, and adjust antidiabetic therapy as needed. Recheck TSH if metformin is started or stopped.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Levothyroxine
Other interactions of Metformin
Frequently asked questions
Can I take Levothyroxine with Metformin?
MediHelp rates the interaction as moderate. Levothyroxine and Metformin can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Correcting hypothyroidism or over-replacing with levothyroxine can raise blood glucose and change glycemic control in patients on metformin. Metformin may also lower TSH in some patients with hypothyroidism.
How serious is the interaction between Levothyroxine and Metformin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor blood glucose and HbA1c when levothyroxine is started or its dose changed, and adjust antidiabetic therapy as needed. Recheck TSH if metformin is started or stopped.
What other medicines interact with Levothyroxine?
MediHelp also lists interactions of Levothyroxine with, for example, Cholestyramine, Sodium iodide i-131, Tiratricol. The full list is on the Levothyroxine page.
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