**Core Concept**
In the management of thyrotoxicosis in pregnancy, it is crucial to consider the safety of both the mother and the fetus. The choice of treatment depends on the gestational age, severity of hyperthyroidism, and potential risks associated with each treatment option.

**Why the Correct Answer is Right**
Radioiodine therapy, specifically radioactive iodine (I-131), is an absolute contraindication in pregnancy due to the risk of fetal thyroid ablation and subsequent cretinism. If I-131 crosses the placenta, it can destroy the fetal thyroid gland, leading to hypothyroidism and potential long-term developmental issues. This risk is particularly concerning in the first trimester, but it is still a contraindication throughout pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Methimazole is generally considered safe in the first trimester, but it is not an absolute contraindication. However, it is still used with caution in pregnancy due to the rare risk of aplasia cutis and other congenital anomalies.

**Option B:** Propranolol is often used to manage symptoms of thyrotoxicosis in pregnancy, such as tachycardia and tremors. It is not an absolute contraindication and can be used safely.

**Option C:** Potassium iodide is used to block the release of thyroid hormone from the thyroid gland. It is not an absolute contraindication in pregnancy and can be used in certain situations, such as during radioactive iodine exposure.

**Clinical Pearl / High-Yield Fact**
In the management of thyrotoxicosis in pregnancy, it is essential to consider the use of thionamides (such as methimazole) in the first trimester, as they are generally considered safer than radioactive iodine. However, radioactive iodine is an absolute contraindication throughout pregnancy due to the risk of fetal thyroid ablation.

**Correct Answer: C. Potassium iodide is used to block the release of thyroid hormone from the thyroid gland.