**Core Concept**
The most sensitive method to diagnose thyroid function is a critical aspect of endocrinology, as it directly affects the treatment and management of thyroid disorders. The optimal diagnostic approach should accurately assess both the production and metabolism of thyroid hormones. This requires a method that can detect even slight alterations in thyroid hormone levels.

**Why the Correct Answer is Right**
The most sensitive method to diagnose thyroid function is the measurement of Thyroid-Stimulating Hormone (TSH). TSH is secreted by the anterior pituitary gland and regulates the production of Thyroid Hormones (TH) by the thyroid gland. TSH levels are extremely sensitive to changes in thyroid hormone levels, making it an ideal marker for assessing thyroid function. A TSH level within the normal range typically indicates euthyroidism, whereas a level outside the normal range suggests hyperthyroidism or hypothyroidism. The TSH assay can detect even slight alterations in thyroid hormone levels, making it the most sensitive method for diagnosing thyroid function.

**Why Each Wrong Option is Incorrect**
**Option A:** Free Thyroxine (FT4) measurement is also an important diagnostic tool for assessing thyroid function, but it is less sensitive than TSH measurement.
**Option B:** Triiodothyronine (T3) measurement is another marker of thyroid function, but it is not as sensitive as TSH measurement and can be affected by non-thyroidal illness.
**Option C:** Thyroid Peroxidase Antibodies (TPOAb) measurement is useful for diagnosing autoimmune thyroiditis, but it is not a direct measure of thyroid function.

**Clinical Pearl / High-Yield Fact**
Remember that TSH measurement is the most sensitive indicator of thyroid function, and a single abnormal TSH level can indicate the presence of a thyroid disorder.

**Correct Answer:** C. TSH measurement is the most sensitive method to diagnose thyroid function.