Synthroid (thyroxine) and estrogen interaction; does this cause weight gain?

No: Being on estrogen may cause you to need more synthroid, (thyroxine) but if your thyroid level is normal, it makes no difference whether you are on estrogen or not.

Interactions: You can use a drug interaction checker whenever you have questions about drug-drug interactions or drug - food interactions. Here is one option (Drugs.Com): https://www.drugs.com/drug_interactions.php Your pharmacist is also an excellent source of information