From the Research
There is no known interaction between tadalafil and synthroid (levothyroxine) that affects absorption, and these medications can generally be taken together without special timing considerations. However, it's essential to note that levothyroxine absorption is affected by many other substances and should ideally be taken on an empty stomach, at least 30-60 minutes before breakfast or other medications 1. Tadalafil, which is used for erectile dysfunction or benign prostatic hyperplasia, works through a completely different mechanism (PDE5 inhibition) than levothyroxine (thyroid hormone replacement), and the two medications have different absorption pathways and don't compete for the same metabolic enzymes in any significant way.
Some key factors that can affect levothyroxine absorption include gastrointestinal disorders, certain foods, and other medications, such as proton pump inhibitors, which can decrease TSH levels in patients with hypothyroidism 2. Additionally, the timing of levothyroxine administration can impact thyroid hormone status, with bedtime dosing potentially offering improved control 3.
If you're taking both tadalafil and levothyroxine, it's crucial to continue following the specific timing instructions for levothyroxine, which is more sensitive to absorption issues than tadalafil. If you notice any changes in thyroid function while taking both medications, consult your healthcare provider, but such interactions would be unlikely. It's also important to note that the diagnosis and treatment of hypothyroidism should be guided by evidence-based practices, including the use of thyroid-stimulating hormone (TSH) levels and thyroxine levels for diagnosis, and levothyroxine monotherapy for treatment, with consideration of alternative therapies in certain cases 4, 5.