From STTM: If I am on thyroid treatment for HYPOthyroidism, will I need to increase it when pregnant?

Yes, say many knowledgeable doctors, since the growing fetus and its tiny brain will need your thyroid hormones for many weeks.  It’s only when your baby is about 20 weeks old does it start to use its own thyroid hormones, but demand for your own thyroid hormones will continue until the baby is born. (And this early need is why some mothers outright “become” hypothyroid while pregnant! The demand overwhelms the thyroid!)The need to increase your thyroid hormone medication, whether natural desiccated thyroid or T3-only, occurs because as estrogen increases with pregnancy, so does the binding of thyroid hormones.

Additionally, if you have the autoimmune version of hypothyroidism called Hashimotos, you’ll want to keep a check on adequately treating your hypothyroidism and antibodies, since some experts state that thyroid antibodies cross the human placenta and could attack your baby’s thyroid.

How much do you increase it? That is something you should talk to your doctor about.