**Core Concept**
The assessment of thyroid function in newborns is crucial for early detection and management of congenital hypothyroidism. Thyroid-stimulating hormone (TSH) measurement is essential for evaluating the thyroid axis and identifying infants at risk of thyroid dysfunction.

**Why the Correct Answer is Right**
TSH measurement in newborns is mandatory to assess the thyroid axis and identify infants with congenital hypothyroidism. Elevated TSH levels indicate primary hypothyroidism, whereas low TSH levels suggest secondary or tertiary hypothyroidism. The American Academy of Pediatrics recommends TSH screening of all newborns within the first few days of life to prevent intellectual disability and growth retardation associated with untreated congenital hypothyroidism.

**Why Each Wrong Option is Incorrect**
* **Option A:** Free thyroxine (FT4) measurement is not mandatory in newborns, as it may not accurately reflect thyroid function, especially in the presence of thyroid-binding globulin abnormalities.
* **Option B:** Triiodothyronine (T3) measurement is not a primary screening test for congenital hypothyroidism, as TSH is more sensitive and specific for assessing thyroid axis.
* **Option D:** Thyroid peroxidase antibody (TPOAb) measurement is not routinely performed in newborns, as it is more relevant for diagnosing autoimmune thyroiditis in older children and adults.

**Clinical Pearl / High-Yield Fact**
Newborn screening for congenital hypothyroidism is a critical public health initiative that has led to a significant decrease in the incidence of intellectual disability and growth retardation associated with untreated hypothyroidism.

**Correct Answer:** C. TSH measurement.