Synthroid (Levothyroxine) and Tadalafil Interaction: What Clinicians and Patients Should Know
Can you take Synthroid with tadalafil?
The combination is not specifically contraindicated in the labels reviewed here. Levothyroxine and tadalafil also have different principal clearance pathways: thyroid-hormone deiodination and hepatic CYP3A4 metabolism, respectively. This supports a low expectation of a metabolism-based interaction; it does not establish that every new symptom has an unrelated cause. Synthroid prescribing information, Cialis prescribing information
Separate the medicine-pair question from the thyroid-disease question. A person with a stable thyroid dose and no symptoms needs a different review from someone with new palpitations after a dose increase or persistent symptoms of hypothyroidism.
Does the timing need to change?
Take Synthroid with water on an empty stomach, 30-60 minutes before breakfast. Its label specifies at least four hours of separation from medicines known to interfere with absorption, including calcium and iron. That instruction does not make every other medicine a four-hour interaction. Tadalafil is not named as an absorption-interfering medicine. Synthroid prescribing information
If taking daily tadalafil later makes the morning routine easier, that is a practical scheduling choice. It should not be described as a proven way to prevent a levothyroxine-tadalafil interaction. For as-needed tadalafil, follow the prescribed schedule rather than adding a thyroid-specific waiting period.
A useful medication list includes supplements as well as prescriptions. Recording a calcium or iron product beside the actual time it is taken is more informative than simply saying that all morning tablets are taken together.
Can a thyroid problem contribute to erectile dysfunction?
Yes, thyroid dysfunction can contribute, although study results differ by population. In a 2008 study, investigators assessed 71 men with thyroid dysfunction, including 44 with hypothyroidism and 27 with hyperthyroidism, alongside controls. Erectile-function questionnaire scores improved after treatment of the thyroid disorder. The study supports reassessment after thyroid treatment; it does not guarantee that every person's erectile dysfunction will resolve. Krassas et al., 2008
A larger 2012 analysis examined two different populations: 3,369 community-dwelling men and 3,203 patients attending a sexual-medicine clinic. It found an association between overt hyperthyroidism and severe erectile dysfunction, but did not find the same association with primary hypothyroidism. Corona et al., 2012
Together, these studies support looking at the individual diagnosis and response to treatment. A percentage from a small thyroid clinic should not be presented as the probability that any man with hypothyroidism will need tadalafil.
If erectile symptoms began near the thyroid diagnosis, record what changes as thyroid treatment takes effect. Persistent symptoms can still have more than one contributor. Reassessing tadalafil's benefit is appropriate; assuming it must either be continued forever or stopped immediately is not.
What monitoring is useful?
For adults with primary hypothyroidism, Synthroid's label describes checking TSH 6-8 weeks after a dose change, then assessing stable treatment every 6-12 months and when clinical status changes. TSH is not the monitoring target for every type of hypothyroidism; pituitary-related disease requires a different interpretation. Synthroid prescribing information
Starting tadalafil does not, by itself, create a separate intensive thyroid-testing schedule. The more useful record connects symptoms with changes:
| What to record | Why it helps |
|---|---|
| Thyroid dose, formulation and date of change | Makes the next thyroid result interpretable |
| Tadalafil schedule and perceived benefit | Distinguishes daily treatment from occasional use |
| Timing of dizziness, palpitations or fatigue | Helps assess whether symptoms preceded or followed a change |
| Other medicines and supplement timing | Identifies issues outside this two-drug pairing |
A blood-pressure reading taken during lightheadedness can be useful if available. A symptom should not be automatically assigned to the thyroid, tadalafil or their combination without reviewing its timing and context.
Which surrounding medicines matter?
Tadalafil must not be combined with nitrates. This is a labeled contraindication, not a boxed warning. Alpha-blockers and blood-pressure medicines also need review because their effects can overlap with tadalafil. Cialis prescribing information
Keeping the full medication list current is especially useful when thyroid and erectile-dysfunction prescriptions come from different clinicians. Include the actual product names, doses and whether each medicine is taken daily or occasionally.
What if thyroid symptoms are not controlled?
An abnormal thyroid result, new palpitations or a markedly slow pulse should prompt reassessment of thyroid treatment and cardiovascular symptoms. A theoretical description of blood-vessel effects cannot determine which drug to change first.
Chest pain, fainting or a very fast or slow heartbeat with symptoms needs urgent assessment. Sudden vision or hearing changes while taking tadalafil also require prompt care. These symptoms should not be managed by simply moving the two tablets farther apart. NHS tadalafil guidance
Frequently asked questions
Do I need to separate tadalafil and Synthroid by four hours?
Will treating hypothyroidism eliminate the need for tadalafil?
Does starting tadalafil automatically require extra TSH tests?
What should I bring to a medication review?
References
- AbbVie. Synthroid prescribing information.
- Eli Lilly. Cialis prescribing information, revised March 2026.
- Krassas et al. Erectile dysfunction in patients with hyper- and hypothyroidism. 2008.
- Corona et al. Thyroid hormones and male sexual function. 2012.
- NHS. Tadalafil. Reviewed May 2026.