I have hashimotos and I’m pregnant. Thyroid goiter completely went Away and then came back all of a sudden. What could cause this? TSH levels are fine

How fine? Hormonal changes in pregnancy include increased function of thyroid gland. HCG, one of the pregnancy hormones, acts as a weak TSH and will increase thyroid function. You ought to be under the care of an obstetrician for the sake of your and your baby's health.
Wish you good health and a healthy baby!