Final answer:
Fine-needle aspiration biopsy is the most cost-effective method for distinguishing between malignant and benign thyroid nodules by analyzing extracted cell samples.
Step-by-step explanation:
The most cost-effective method of distinguishing a malignant from a benign thyroid nodule is fine-needle aspiration biopsy. This procedure involves using a thin needle to extract cells from the thyroid nodule. The cells are then examined under a microscope to determine whether they are cancerous. In contrast, thyroid scans using radioactive iodine may show which parts of the thyroid are more active and can determine if the nodule is functioning ("hot") or non-functioning ("cold"), but they are more expensive and less definitive in diagnosis compared to biopsy. An ultrasound can help in assessing the nodule's characteristics and guide the biopsy but cannot differentiate between benign and malignant tissue. Lastly, thyroid function tests assess the thyroid's hormone production level, which, although important, do not directly indicate whether a nodule is malignant or benign.