Levothyroxine and Simvastatin: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 4, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Levothyroxine
Simvastatin
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
If you take levothyroxine for a low or underactive thyroid and you also start simvastatin for cholesterol, your thyroid medicine may not work quite as well. A few case reports found that simvastatin can speed up how fast your body breaks down levothyroxine, so your thyroid levels can drift out of range.
The good news is this is easy to keep an eye on. Your doctor can check your thyroid with a simple blood test called TSH and adjust your levothyroxine dose if needed. Keep taking both medicines as prescribed, and let your pharmacist or doctor know you are on both so they can watch your levels.
Effect: Possible decreased levothyroxine efficacy with concurrent simvastatin.
Mechanism: Postulated CYP3A4 induction by simvastatin accelerating levothyroxine metabolism, reducing exposure and raising TSH. Levothyroxine is a narrow-therapeutic-index agent, so even modest shifts are clinically relevant.
- Direction: Reduced thyroid hormone effect (potential undertreatment).
- Onset: Delayed.
- Evidence: Probable, based on three case reports of increased TSH.
- Management: Monitor TSH and clinical response after initiating or changing simvastatin; titrate levothyroxine dose as individualized by the care team.
What happens
Decreased levothyroxine efficacy
Interaction Deep Dive
Elevations in thyroid stimulating hormone (TSH) were reported in three cases where levothyroxine and simvastatin were taken together. One proposed explanation for this interaction is that simvastatin drives excess CYP3A4 production in the liver, which in turn speeds up levothyroxine metabolism21. When these drugs are given concurrently, it may be advisable to watch the patient for an adequate therapeutic response to levothyroxine.
Why it happens (mechanism)
Increased levothyroxine metabolism
How to manage this interaction
This interaction is manageable with routine follow-up. Keep taking both medicines exactly as prescribed unless your care team tells you otherwise.
- Your team may check your TSH (a thyroid blood test) a bit more closely after you start or change simvastatin.
- Your levothyroxine dose may need to be adjusted and individualized based on those results.
- Tell your pharmacist and prescriber you take both drugs.
- Report symptoms of an underactive thyroid, such as fatigue, weight gain, feeling cold, or low mood.
Management is individual — confirm any change with your pharmacist or prescriber.
Literature reports
3 reports — tap to read
a) An 85-year-old woman receiving simvastatin 80 mg, levothyroxine 100 mcg, and her usual antihypertensive medications developed hypothyroidism and rhabdomyolysis 3 weeks after her simvastatin dose was raised to better control lipids. The patient, whose extensive medical history included osteoporosis, hypertension, hypercholesterolemia, mild chronic renal impairment, and hypothyroidism, came to the emergency room following 7 days of muscle pain, worsening lower limb weakness, and inability to bear weight. She had mild renal and hepatic impairment along with a creatinine kinase (CK) value of 2228, a free thyroxine (T4) level at the lower limit of normal, and an elevated thyroid stimulating hormone (TSH) of 28.6 International Units/L. MRI of the brain and spinal cord revealed no intracranial abnormalities. Simvastatin was stopped promptly while the levothyroxine dose was kept at 100 mcg. Soon after admission, CK rose to 32,000 with ongoing decline in renal and hepatic function. Muscle biopsy showed findings consistent with a statin-induced myopathy. Over the following month, renal and hepatic function together with thyroid function tests gradually normalized. The patient was discharged after 5 weeks without any long-term adverse effects. The proposed mechanism is that simvastatin causes excess formation of CYP3A4 in the liver, thereby speeding up levothyroxine metabolism 1.
b) A 75-year-old woman being treated with levothyroxine 800 mcg/week, alendronate 10 mg/day, and calcium supplements developed fatigue, abdominal pain, and rising thyroid stimulating hormone (TSH) levels after she was started on simvastatin 10 mg/day for hyperlipidemia. Following 4 months of simvastatin therapy, her levothyroxine dose was raised to 900 mcg/week because of increasing TSH levels (47.83 micro-International Units/liter), with free thyroxine (T4) levels at the lower limit of normal. After 16 days on the higher levothyroxine dose, her symptoms had not improved (TSH level, 28.63 micro-International Units/liter). Liver function tests rose to slightly above normal during the period of hypothyroidism. When simvastatin was subsequently discontinued, her symptoms resolved and the levothyroxine dose was reduced to the original 800 mcg/week. During the following month, the patient's TSH level returned to and stayed within normal limits. Bezafibrate therapy was started with no adverse effects. The proposed mechanism is that simvastatin causes excess formation of CYP3A4 in the liver, thereby speeding up levothyroxine metabolism 2.
c) An 81-year-old man had rising thyroid stimulating hormone (TSH) after being started on levothyroxine for elevated TSH levels and, shortly afterward, being started on simvastatin. His concurrent medications included vitamin D, allopurinol, isosorbide mononitrate, felodipine, alprazolam, furosemide, tamsulosin, clopidogrel, and erythropoietin. He had previously been treated successfully with levothyroxine for an episode of amiodarone-induced hypothyroidism, and he had stable renal failure with plasma creatinine of approximately 4 mg/dL, together with chronic anemia, following a unilateral nephrectomy. Two years after his initial levothyroxine therapy, TSH levels of 11.76 micro-International Units/Liter with free thyroxine (T4) levels at the lower end of normal led to reinitiation of levothyroxine 50 mcg/day. Simvastatin 10 mg/day was added to his regimen soon after. In the weeks after simvastatin was started, his TSH level kept rising to 23.9 micro-International Units/Liter. Simvastatin was immediately stopped and, within 4 weeks, TSH levels fell to within the normal range with no change in the levothyroxine dose. He subsequently began pravastatin for his elevated cholesterol level without any adverse effects on his thyroid status 2.
Common questions
Can I take Levothyroxine and Simvastatin together?
Simvastatin may slightly weaken your levothyroxine, so your thyroid can become underactive; keep taking both and let your doctor monitor your TSH and adjust the dose if needed. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Levothyroxine and Simvastatin interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How is the Levothyroxine and Simvastatin interaction managed?
This interaction is manageable with routine follow-up. Keep taking both medicines exactly as prescribed unless your care team tells you otherwise. Your team may check your TSH (a thyroid blood test) a bit more closely after you start or change simvastatin. Your levothyroxine dose may need to be adjusted and individualized based on those results. Tell your pharmacist and prescriber you take both dr… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
Questions for your pharmacist
- Does my dose of Levothyroxine or Simvastatin need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (2)
- Kiernan TJ, Rochford M, & McDermott JH: Simvastatin induced rhabdomyolysis and an important clinical link with hypothyroidism. Int J Cardiol 2007; 119(3):374-376. PubMed
- Kisch E & Segall HS: Interaction between simvastatin and L-thyroxine. Ann Intern Med 2005; 143(7):547.
Keep reading about Levothyroxine
Keep reading about Simvastatin
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