Synthroid is the brand name; the name of the medicine is levothyroxine. It can safely be given during pregnancy. Adequate studies in pregnant women have failed to show a risk to the fetus in the first three months (the most critical period) and there is no risk in later months.
In fact not taking levothyroxine is worse because of the danger of abnormality due to hypothyroidism.
There are no food alternatives to levothyroxine. Even if they existed, I would urge you to continue to take your prescribed medication. There is no need to worry at all.