**Core Concept**
Thyroid function tests (TFTs) are crucial in diagnosing and managing hypothyroidism. Among various TFTs, one investigation stands out as the most informative and commonly used due to its high sensitivity and specificity.

**Why the Correct Answer is Right**
The most informative and commonly used investigation in hypothyroidism is the measurement of Thyroid Stimulating Hormone (TSH). TSH is produced by the pituitary gland and stimulates the thyroid gland to produce thyroid hormones (T3 and T4). In primary hypothyroidism, the thyroid gland does not produce enough thyroid hormones, leading to an increase in TSH production. Elevated TSH levels are a sensitive indicator of primary hypothyroidism, making it the most informative investigation. Additionally, TSH levels can help differentiate between primary and secondary hypothyroidism.

**Why Each Wrong Option is Incorrect**
**Option A:** Free Thyroxine (FT4) measurement is useful but less sensitive than TSH measurement in detecting mild hypothyroidism.
**Option B:** Triiodothyronine (T3) measurement is not commonly used due to its short half-life and variability in production.
**Option C:** Thyroid Antibody measurement is useful in diagnosing autoimmune thyroiditis but not as commonly used as TSH measurement in primary hypothyroidism.

**Clinical Pearl / High-Yield Fact**
TSH measurement is a sensitive indicator of primary hypothyroidism, and a single elevated TSH value is sufficient to diagnose hypothyroidism, especially in patients with symptoms and a low FT4 level.

**Correct Answer: C. Thyroid Stimulating Hormone (TSH) measurement.**