A case of solitary thyroid nodule, Investigation of choice is:
**Core Concept**
Fine-needle aspiration cytology (FNAC) is the preferred initial investigation for solitary thyroid nodules, especially when they are larger than 1 cm or show suspicious ultrasonographic features. This is because FNAC allows for accurate diagnosis of malignant or benign nature of the nodule, thereby guiding further management.
**Why the Correct Answer is Right**
FNAC involves inserting a thin needle into the nodule under ultrasound guidance to collect a sample of cells for examination. This diagnostic procedure is highly sensitive and specific for differentiating between malignant and benign thyroid nodules. The cytological findings can be classified into several categories, including non-diagnostic, benign, atypia of undetermined significance, suspicious for malignancy, and malignant. Based on the cytological diagnosis, further management can be planned, which may include repeat FNAC, core needle biopsy, or surgical excision.
**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound alone may not provide a definitive diagnosis, especially in cases where the nodule has a non-characteristic appearance or when there are atypical features. While ultrasound is useful for evaluating the size, shape, and vascularity of the nodule, it may not be sufficient for making a definitive diagnosis of malignancy.
**Option B:** Serum calcitonin levels are typically elevated in medullary thyroid carcinoma, but this test is not the investigation of choice for solitary thyroid nodules. Calcitonin levels can also be elevated in other conditions, such as thyroiditis or lymphoma, making this test less specific for diagnosing thyroid cancer.
**Option C:** Thyroid-stimulating hormone (TSH) levels are essential for evaluating thyroid function, but they do not provide information about the nature of the thyroid nodule itself. A solitary thyroid nodule may be associated with elevated or suppressed TSH levels, but this information does not guide the diagnosis or management of the nodule.
**Clinical Pearl / High-Yield Fact**
When performing FNAC, it is essential to use a 22-25 gauge needle and to aspirate multiple samples from different areas of the nodule to ensure an accurate diagnosis. This approach can improve the sensitivity and specificity of FNAC and reduce the likelihood of false-negative or false-positive results.
**Correct Answer: C. Thyroid-stimulating hormone (TSH) levels are essential for evaluating thyroid function, but they do not provide information about the nature of the thyroid nodule itself. A solitary thyroid nodule may be associated with elevated or suppressed TSH levels, but this information does not guide the diagnosis or management of the nodule.**