The ultrasound features of a thyroid nodule—hypoechoic texture, irregular margins, and microcalcifications—are highly suspicious for malignancy. Fine-needle aspiration (FNA) biopsy is the most accurate and minimally invasive method to obtain cytological diagnosis, guiding further management. Early identification of malignant cells allows for timely surgical intervention if needed. Other options like thyroid hormone therapy or observation do not provide diagnostic clarity, and radioactive iodine scan is less sensitive for initial evaluation of suspicious nodules. Therefore, FNA is the next essential diagnostic step to confirm or exclude thyroid cancer.

Reference: Harrison's Principles of Internal Medicine, Kasper et al., 20th Edition.