**Core Concept**
Thyroid function tests (TFTs) are crucial for diagnosing thyroid-related disorders. The primary marker used to assess thyroid function is the thyroid-stimulating hormone (TSH), which regulates the production of thyroid hormones.

**Why the Correct Answer is Right**
TSH is produced by the pituitary gland and stimulates the thyroid gland to produce thyroid hormones, triiodothyronine (T3) and thyroxine (T4). An elevated TSH level indicates hypothyroidism, while a decreased TSH level suggests hyperthyroidism. TSH is the most sensitive marker for assessing thyroid function and is often used as the initial test in the evaluation of thyroid disorders. It is also useful for monitoring the effectiveness of thyroid hormone replacement therapy.

**Why Each Wrong Option is Incorrect**
**Option A:** T3 is an active thyroid hormone, but it is not the best marker for diagnosing thyroid-related disorders because its levels can fluctuate due to various factors, making it less reliable for diagnosis.

**Option B:** T4 is another thyroid hormone, but its levels can be affected by factors such as liver disease, making it less accurate for diagnosing thyroid disorders.

**Option D:** Thyroglobulin is a protein produced by the thyroid gland, but its levels are not a reliable marker for assessing thyroid function or diagnosing thyroid-related disorders.

**Clinical Pearl / High-Yield Fact**
When interpreting TFTs, it's essential to remember that TSH is the primary marker for assessing thyroid function, and a single abnormal value should not be considered in isolation. A combination of TSH, T3, and T4 levels should be evaluated to make an accurate diagnosis.

✓ Correct Answer: C. TSH