What is the classic sign for thyroid neoplasia?

For thyroid neoplasia, the classic sign observed on 131Iodine uptake studies is decreased uptake. This is particularly relevant in the context of certain types of thyroid cancer, especially thyroid follicular carcinoma and medullary thyroid carcinoma, which often exhibit reduced iodine uptake due to their decreased functional thyroid tissue activity.

In general, thyroid nodules that are functioning, such as those seen in hyperthyroidism, would take up iodine and demonstrate increased uptake. Conversely, neoplastic lesions that are less functional or are non-functional will typically show a decrease in iodine uptake. Therefore, when a thyroid nodule presents with reduced isotope uptake, it raises suspicion for a neoplastic process.

Other signs such as the presence of calcitonin may be indicative of medullary thyroid carcinoma but are not a universal characteristic of all thyroid neoplasias. Palpable cervical lymphadenopathy may suggest metastasis or regional involvement in thyroid cancer, but it is not specific or a classic sign of the underlying neoplasm itself. Increased uptake on iodine studies is not a characteristic of neoplasia, thereby reinforcing that decreased uptake is indeed the classic sign you'd look for when evaluating potential thyroid malignancies.