In the second and third trimester, this risk is diminished and methimazole is preferred to avoid the risk of liver complications from PTU in the mother.The primary effect on the fetus from transplacental passage of PTU is the production of a mild hypothyroidism when the drug is used close to term. This usually resolves within a few days without treatment. The hypothyroid state may be observed as a goiter in the newborn, and is the result of increased levels of fetal pituitary thyrotropin. The incidence of fetal goiter after PTU treatment in reported cases is approximately 12%. Mechanism of action
Thyroid
PTU inhibits the enzyme thyroperoxidase, which normally acts in thyroid hormone synthesis by oxidizing the anion iodide (I−) to iodine (I0), facilitating iodines addition to tyrosine residues on the hormone precursor thyroglobulin. This is one of the essential steps in the formation of thyroxine (T4).PTU does not inhibit the action of the sodium-dependent iodide transporter located on follicular cells basolateral membranes. Inhibition of this step requires competitive inhibitors, such as perchlorate and thiocyanate. T3/T4 target tissues
PTU also acts by inhibiting the enzyme 5-deiodinase (tetraiodothyronine 5 deiodinase), which converts T4 to the more active form T3. (This is in contrast to methimazole, which shares propylthiouracils central mechanism, but not its peripheral one.) It is important to recognize that these enzymes only work on the conjugated tyrosine molecules of T3 and T4: a completely different enzyme family is responsible for the deiodinase activity of iodized single tyrosine molecules within the thyroid follicular cells.
Propylthiouracil (PTU) is a medication used to treat hyperthyroidism. This includes hyperthyroidism due to  Graves disease and toxic multinodular goiter. In a thyrotoxic crisis it is generally more effective than methimazole. Otherwise it is typically only used when methimazole, surgery, and radioactive iodine is not possible. It is taken by mouth.Common side effects include itchiness, hair loss, parotid swelling, vomiting, muscle pains, numbness, and headache. Other severe side effects include liver problems and low blood cell counts. Use during pregnancy may harm the baby. Propylthiouracil is in the antithyroid family of medications. It works by decreasing the amount of thyroid hormone produced by the thyroid gland and blocking the conversion of thyroxine (T4) to triiodothyronine (T3).Propylthiouracil came into medical use in the 1940s. It is on the World Health Organizations List of Essential Medicines. Side effects
Propylthiouracil is generally well tolerated, with side effects occurring in one of every 100 patients. The most common side effects are related to the skin and include rash, itching, hives, abnormal hair loss, and skin pigmentation. Other common side effects are swelling, nausea, vomiting, heartburn, loss of taste, joint or muscle aches, numbness and headache, allergic reactions, and hair whitening.Its notable side effects include a risk of agranulocytosis and aplastic anemia. On 3 June 2009, the FDA published an alert "notifying healthcare professionals of the risk of serious liver injury, including liver failure and death, with the use of propylthiouracil."