**Core Concept**
Thyroid function tests (TFTs) are crucial for diagnosing and managing hypothyroidism. These tests measure various aspects of thyroid hormone production and metabolism, helping clinicians determine the underlying cause of hypothyroidism.

**Why the Correct Answer is Right**
The most informative and commonly used investigation for hypothyroidism is the **Free Thyroxine (FT4) test**. This test measures the level of free thyroxine (T4) in the blood, which is the biologically active form of thyroid hormone. Elevated T4 levels indicate primary hypothyroidism, while low T4 levels suggest secondary or tertiary hypothyroidism. Additionally, the **Thyroid-Stimulating Hormone (TSH) test** is often used in conjunction with FT4 to assess thyroid function. However, FT4 is the most direct measure of thyroid hormone levels.

**Why Each Wrong Option is Incorrect**
* **Option A:** T3 (Triiodothyronine) test is not the most informative investigation for hypothyroidism, as T3 levels can fluctuate widely due to various factors, including non-thyroidal illness.
* **Option B:** **Free Triiodothyronine (FT3) test** is not the most commonly used test for hypothyroidism, although it can be useful in certain situations, such as assessing thyroid function in patients with non-thyroidal illness.
* **Option C:** **Thyroid Antibody tests** are useful for diagnosing autoimmune thyroiditis, but they are not the most informative or commonly used tests for hypothyroidism.

**Clinical Pearl / High-Yield Fact**
When interpreting thyroid function tests, it's essential to consider the patient's clinical context, including symptoms, medical history, and laboratory results from other tests. A high TSH level with a low FT4 level suggests primary hypothyroidism, while a high TSH level with a normal FT4 level may indicate subclinical hypothyroidism.

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