Hello and thank you for your question. I would also like to thank you for being careful about your health. It sounds like you are trying to prevent problems, and both the multivitamin and the Culturelle are a good idea. The good news is that there is no problem taking Culturelle, synthroid and multivitamin on the same day. The synthroid should be taken with water alone on an empty stomach. The multivitamin should be taken with food to minimize stomach upset. The culturelle can be taken with or without food - so the easiest thing to do would be to take the culturelle and the vitamin at the same time. They will not interfere with one another. I hope that this has answered your question. If you have any further questions, please check back. In the meanwhile, good job staying healthy!