If you're hypothyroid, zinc, manganese, selenium, iodine, sodium, potassium, chromium, (sometimes iron), B-complex, vitamin C, and less fat can all boost thyroid function.
High amounts of calcium and magnesium, copper, choline, inositol, vitamin D, and a high fat diet will tend to slow down the thyroid.
If you're not hypothyroid and using synthroid to lose weight you may impair your health. It's possible your weight problems may be due to some other variable. Thyroid drugs are catabolic, which isn't a good thing if you're trying to gain/maintain muscle.