Synthroid (Levothyroxine) and Tadalafil Interaction: What Clinicians and Patients Should Know
Levothyroxine (brand name Synthroid, among others) is a synthetic thyroid hormone used to treat hypothyroidism. Tadalafil (brand name Cialis) is a phosphodiesterase type 5 (PDE5) inhibitor approved for erectile dysfunction, benign prostatic hyperplasia, and pulmonary arterial hypertension. This page covers the oral tablet forms of both drugs, not compounded or combination formulations.
The short answer
There is no known pharmacokinetic drug-drug interaction between levothyroxine and tadalafil. They are not metabolized by the same enzymes, do not compete for the same transporters, and neither drug's FDA prescribing information names the other as a contraindication or dose-adjustment trigger. What deserves more attention is a disease-drug relationship: untreated or under-treated hypothyroidism can itself cause erectile dysfunction and can alter how a patient's blood pressure and heart rate respond to a vasodilator like tadalafil. That distinction, not a pharmacy database flag, is the useful clinical question here.
Why this pairing raises questions at all
Levothyroxine has a narrow therapeutic index and a well-documented list of absorption interactions with calcium carbonate, iron salts, and proton pump inhibitors. Tadalafil carries an FDA boxed warning about concomitant nitrate use because of the risk of severe hypotension. Patients and clinicians reasonably wonder whether combining a narrow-index hormone with a vasodilator needs the same caution. Based on the mechanisms below, it does not, at least not for a pharmacokinetic reason.
Do the two drugs share any metabolic pathway?
Levothyroxine is absorbed mainly in the small intestine and is cleared through sequential deiodination (type 1, 2, and 3 deiodinase enzymes) along with glucuronidation and sulfation. It is not a substrate of the cytochrome P450 system in any way that is clinically established. This is described in levothyroxine's FDA prescribing information.
Tadalafil is metabolized predominantly by CYP3A4, with a minor contribution from CYP3A5. The FDA label for tadalafil lists interactions with strong CYP3A4 inhibitors (for example, ketoconazole, ritonavir) and inducers (for example, rifampin), and separately warns against nitrate co-administration. Levothyroxine does not appear anywhere in that interaction section.
Because the two drugs are cleared by different enzyme systems and neither is a clinically significant inhibitor or inducer of the other's pathway, there is no established mechanism for one drug to raise or lower blood levels of the other. This is a mechanistic conclusion drawn from each drug's labeled metabolism, not a claim backed by a dedicated interaction trial, and no such trial appears to have been conducted specifically in this drug pair.
Is there a meaningful pharmacodynamic overlap?
The absence of a pharmacokinetic interaction does not mean the two conditions are unrelated. Thyroid hormone has direct cardiovascular effects: it influences heart rate, cardiac output, and vascular resistance. In overt hypothyroidism, cardiac output can fall and diastolic blood pressure can rise as peripheral vascular resistance increases; this is well established in thyroid physiology literature, though the exact magnitude varies by patient and severity of disease and should not be quoted as a fixed number for an individual patient.
Tadalafil produces a modest average blood pressure reduction in clinical trials, generally described in single digits of mmHg in normotensive subjects per its FDA label. In a patient whose hypothyroidism is not yet controlled and whose diastolic pressure is elevated, adding a vasodilator could plausibly produce a more noticeable hemodynamic effect. This is a disease-drug interaction, not a drug-drug interaction: the cardiovascular backdrop created by uncontrolled thyroid disease changes how a vasodilator behaves in that patient, not how the two drugs interact chemically.
Can hypothyroidism itself cause the erectile dysfunction being treated?
This is the question most likely to change a treatment plan. Sexual dysfunction, including erectile dysfunction, has long been reported as a symptom of both hypothyroidism and hyperthyroidism in the endocrinology literature, and case series and small studies have reported improvement in erectile function after thyroid hormone levels are normalized, in some patients without any PDE5 inhibitor. The proposed mechanism involves thyroid hormone's role in nitric oxide signaling within vascular and smooth muscle tissue, including the corpus cavernosum.
The exact prevalence figures and improvement rates that circulate for this association (for example, specific percentages of hypothyroid men affected, or the percentage who improve after treatment) vary across studies and sample sizes, and the primary papers behind those figures could not be independently verified for this draft. Rather than repeat an unverified precise number, the practical takeaway is qualitative and still clinically useful: a man who is newly diagnosed with hypothyroidism and also has erectile dysfunction should have his thyroid status corrected and reassessed before assuming the erectile dysfunction requires indefinite PDE5 inhibitor therapy. Some patients improve with thyroid treatment alone; others do not, and thyroid status is one of several contributors to erectile dysfunction alongside vascular, neurologic, psychological, and medication-related causes.
Does tadalafil interfere with levothyroxine absorption or dosing?
No mechanism supports this. Levothyroxine absorption is reduced by calcium carbonate, iron salts, aluminum-containing antacids, bile acid sequestrants, sucralfate, and proton pump inhibitors, which is why the FDA label instructs patients to take it on an empty stomach, 30 to 60 minutes before food or other medications. Tadalafil does not alter gastric pH, does not chelate minerals, and does not bind thyroid hormone in the gut. There is no published evidence that tadalafil reduces levothyroxine bioavailability.
For a patient on daily low-dose tadalafil (2.5 mg or 5 mg), taking it 30 to 60 minutes after the morning levothyroxine dose is a reasonable, low-effort precaution consistent with general levothyroxine spacing advice, though it is not a documented requirement specific to tadalafil. For on-demand tadalafil (10 mg or 20 mg), which is typically taken separately from the morning medication routine, timing relative to levothyroxine is not a practical concern.
What do interaction checkers actually show?
Standard drug interaction reference tools generally do not list a significant interaction between levothyroxine and tadalafil, which is consistent with the absence of a shared enzyme or transporter pathway described above. This statement reflects general pharmacology reasoning and common interaction-checker behavior rather than a specific, independently verified database export, and a pharmacist or clinician should confirm current output in the tool they use, since interaction databases update over time (checked as of 2026-05).
Monitoring conversation guide for a patient on both drugs
- Confirm TSH is at or trending toward the reference range before crediting or blaming tadalafil for any symptom change. Guideline bodies such as the American Thyroid Association recommend rechecking TSH roughly 4 to 8 weeks after any levothyroxine dose or formulation change.
- Check baseline blood pressure before starting tadalafil, and periodically afterward if the patient has known or suspected uncontrolled hypothyroidism.
- If erectile dysfunction was diagnosed around the same time as hypothyroidism, plan a reassessment of erectile function after TSH normalizes, before assuming tadalafil must be continued indefinitely.
- Review the full medication list rather than isolating this one pair: calcium supplements, iron, and antacids remain the more consequential levothyroxine interactions in most patients, and alpha-blockers combined with tadalafil carry a real additive hypotension risk that is unrelated to thyroid status.
Special populations and confounding polypharmacy
Hypothyroidism and erectile dysfunction both become more common with age, so co-prescription of levothyroxine and tadalafil is common in practice. In patients with coronary artery disease, thyroid status matters more than the levothyroxine-tadalafil pairing itself: undertreated hypothyroidism has been linked to adverse cardiovascular outcomes in observational research, and tadalafil is contraindicated with nitrates regardless of thyroid status. The Princeton III Consensus framework is the guideline reference most clinicians use to judge cardiovascular readiness for PDE5 inhibitor therapy in general, separate from thyroid considerations.
The more common real-world risk is not the levothyroxine-tadalafil pair but the other drugs surrounding it: alpha-blockers (tamsulosin, doxazosin) combined with tadalafil can cause additive hypotension; calcium supplements taken for bone health directly impair levothyroxine absorption if not separated. A full medication reconciliation matters more than scrutinizing this specific pair.
When to slow down before combining these drugs
Two situations call for caution, and in both cases the underlying problem is thyroid status, not a drug-drug interaction:
Uncontrolled hypothyroidism with a notably slow heart rate. Significant bradycardia in overt hypothyyroidism reflects reduced adrenergic tone. Adding a vasodilator before thyroid hormone is corrected could plausibly produce orthostatic symptoms. Correct the thyroid deficit first, or coordinate closely between the prescribing clinicians.
Recent overcorrection or thyrotoxicosis. Excess thyroid hormone raises heart rate and myocardial oxygen demand. Tadalafil-driven vasodilation in a thyrotoxic or recently over-replaced patient could compound tachycardia-related symptoms. This is managed through levothyroxine dose titration, not by avoiding tadalafil outright.
Anyone who develops chest pain, fainting, a very fast or very slow heart rate, or a sudden vision or hearing change while on either medication needs urgent evaluation rather than waiting for a routine follow-up.
Evidence-status assessment: levothyroxine plus tadalafil
| Claim | Status | Basis | What to verify before relying on it |
|---|---|---|---|
| No shared CYP or deiodinase pathway between the two drugs | Established | FDA labels for both drugs describe non-overlapping metabolism | Confirm current label language has not changed |
| Neither FDA label lists the other drug as an interaction | Established | Direct label review | Re-check label version/date if prescribing years from now |
| Tadalafil does not reduce levothyroxine absorption | Plausible, not directly studied | Mechanistic reasoning (no chelation, no gastric pH change); no dedicated absorption trial identified | Ask a pharmacist to check for any newer absorption study |
| Hypothyroidism can cause or worsen erectile dysfunction | Established as an association; magnitude uncertain | Long-standing endocrinology literature on thyroid-sexual function links | Verify specific prevalence and resolution percentages against the primary study before quoting a number |
| Adding tadalafil in uncontrolled hypothyroidism could produce more pronounced hypotension | Plausible, not directly tested in this pair | General thyroid cardiovascular physiology plus tadalafil's known modest hypotensive effect | No dedicated combination trial exists; treat as theoretical until studied |
| Interaction databases show no flagged interaction | Reasonably well supported | Consistent with mechanistic separation above | Confirm current output in the tool actually used, since databases update |
| Exact percentage of hypothyroid men with ED, or percentage who improve after treatment | Not established at the precision often quoted | Underlying primary studies could not be verified for this draft | Do not cite a specific percentage without pulling the original paper |
What this page does not establish
This review does not establish a dosing schedule for an individual patient, does not replace an endocrinologist's or urologist's assessment, and does not confirm the specific study-level statistics sometimes cited for thyroid-related erectile dysfunction. It also does not evaluate compounded or non-oral formulations of either drug. Anyone making a treatment decision based on the thyroid-ED link described above should ask their clinician to point to the specific primary study behind any number quoted, rather than accept a secondhand statistic.
Frequently asked questions
Can I take Synthroid with tadalafil?
Does tadalafil affect how well my levothyroxine works?
Should I space out Synthroid and tadalafil during the day?
Can an underactive thyroid cause erectile dysfunction?
Will I still need tadalafil once my thyroid levels are normal?
What actually interacts with levothyroxine?
References
- FDA. Cialis (tadalafil) prescribing information. https://accessdata.fda.gov/drugsatfda_docs/label/2011/021368s20s21lbl.pdf
Other claims in this article regarding thyroid-related sexual dysfunction prevalence, cardiovascular outcome statistics, and specific study sample sizes reference general endocrinology and urology literature that could not be independently verified against the original papers for this draft. A clinician or medical librarian should confirm the primary sources before those figures are quoted publicly.