**Core Concept**
Thyroid function tests (TFTs) are essential in evaluating thyroid nodules to assess thyroid hormone levels and screen for thyroid dysfunction, which can guide further management and investigations.

**Why the Correct Answer is Right**
Thyroid function tests (TFTs) are the initial preferred investigation for thyroid nodules because they help identify primary hypothyroidism or hyperthyroidism, which is crucial in planning further management. TFTs include measurements of free thyroxine (FT4), free triiodothyronine (FT3), thyroid-stimulating hormone (TSH), and sometimes thyrotropin-releasing hormone (TRH). These tests can also help differentiate between euthyroid and non-euthyroid states, which affects the choice of subsequent investigations. In euthyroid patients, further evaluation with imaging studies or fine-needle aspiration cytology (FNAC) may be considered.

**Why Each Wrong Option is Incorrect**

* **Option A:** FNAC is a useful investigation for evaluating thyroid nodules but is not the initial preferred investigation. It is typically performed after TFTs have been done and the patient is found to be euthyroid or has thyroid dysfunction that requires further evaluation.
* **Option B:** Radionucleide tests, such as radioactive iodine uptake (RAIU) or thyroid scan, are useful in evaluating thyroid nodules but are not the initial preferred investigation. They are usually performed after TFTs have been done and the patient is found to be euthyroid or has thyroid dysfunction that requires further evaluation.
* **Option D:** Ultrasound (USG) of the thyroid gland is a useful imaging modality for evaluating thyroid nodules but is not the initial preferred investigation. USG is often used in conjunction with TFTs to evaluate the size, number, and characteristics of thyroid nodules.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that thyroid function tests (TFTs) are the initial preferred investigation for thyroid nodules because they help identify primary thyroid dysfunction, which can affect the choice of subsequent investigations and guide management.

**✓ Correct Answer: C. Thyroid function test.**