Hello and thank you for your question!
Yes it is possible to take Tums during pregnancy, but there should be a 4 hour gap between your synthroid and Tums. My preference, for moms, is Gaviscon (better effectiveness for reflux).
Other tips to apply to reduce reflux:
1- avoid eating and drinking anything other than water 3 hours before bedtime
2- If possible, raise the head of your bed (10-15cm)
3- Lie on your left side when you sleep
4- Avoid certain foods (very fatty foods, chocolate, spices, mint, soft drinks, tomato juice, coffee, tea)
I hope this helps you a little :)
Have a good evening,
Emmanuelle