Final answer:
Propylthiouracil (PTU) is the preferred treatment for Graves' disease in a pregnant patient because it safely controls the production of thyroid hormones, while alternatives like Methimazole and radioactive iodine are contraindicated during pregnancy.
Step-by-step explanation:
The preferred treatment for a 33-year-old primigravida patient with Graves' disease is the administration of antithyroid drugs, specifically Propylthiouracil (PTU). PTU is favored during the first trimester of pregnancy due to the increased risk of teratogenic effects associated with Methimazole. Radioactive iodine therapy is generally contraindicated during pregnancy as it may harm the developing fetus. Instead, PTU is used to inhibit the enzyme thyroperoxidase, which in turn helps control the overproduction of thyroid hormones.