Norethisterone (regestrone) is the usual medicine doctors use to delay periods for travel, and it is generally safe for a short course. At 45, cycles stretching to day 30 or 35 can simply be your body moving toward perimenopause, which is common.
Next Steps
Please see your gynaecologist this week so they can confirm the right starting date and dose for your trip from the 15th, and mention that you're on
thyroid medication.
Health Tips
Don't start the tablet on your own without your doctor confirming the dose, and come off it only as they advise once you're back.