**Question:** After thyroidectomy for medullary carcinoma of thyroid, which is important for determining recurrence of tumor?

**Core Concept:** Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumor that originates from the parafollicular C cells of the thyroid gland. Recurrence after thyroidectomy is a significant concern, and several markers can be used to monitor for tumor recurrence.

**Why the Correct Answer is Right:** Carcinoembryonic antigen (CEA) is a glycoprotein that is elevated in various malignancies, including MTC. Monitoring CEA levels can be useful in detecting tumor recurrence after thyroidectomy for MTC.

**Why Each Wrong Option is Incorrect:**
A. Calcitonin (CT) is a polypeptide hormone that is secreted by the parafollicular C cells of the thyroid gland. While CT is specific for MTC, monitoring CEA levels can be more sensitive for detecting recurrence, as CEA levels may increase even without tumor recurrence due to other factors.
B. Thyroglobulin (Tg) is a glycoprotein produced by thyroid follicular cells. Tg is not specific for MTC and is not commonly used for monitoring MTC recurrence.
C. Thyroid-stimulating hormone (TSH) is a pituitary hormone that stimulates thyroid gland function. Monitoring TSH levels is not useful for detecting MTC recurrence, as TSH is not specific for thyroid cancer and does not indicate tumor recurrence.
D. CEA is a biomarker for detecting MTC recurrence, but monitoring CEA levels alongside CT can provide a more accurate evaluation of tumor recurrence.

**Core Concept:** Calcitonin (CT) is a specific biomarker for medullary thyroid carcinoma (MTC), originating from the parafollicular C cells of the thyroid gland.

**Why the Correct Answer is Right:** Monitoring CT levels is crucial for assessing MTC recurrence, as it reflects the presence of functioning C cells and is highly specific to MTC.

**Why Each Wrong Option is Incorrect:**
A. Thyroglobulin (Tg) is a glycoprotein produced by thyroid follicular cells, not specific to MTC, and hence is not suitable for monitoring MTC recurrence.
B. Thyroid-stimulating hormone (TSH) is a pituitary hormone that stimulates thyroid gland function. Monitoring TSH levels is not useful in detecting MTC recurrence, as TSH is not specific for thyroid cancer and does not indicate tumor recurrence.
C. Carcinoembryonic antigen (CEA) is a biomarker for detecting MTC recurrence, but monitoring CEA levels alongside CT can provide a more accurate evaluation of tumor recurrence, as CEA is less specific than CT for MTC.
D. Monitoring CEA levels can be useful in detecting MTC recurrence, but combining it with CT offers a more comprehensive evaluation of tumor recurrence, as CT is highly specific to MTC.

**Core Concept:** Calcitonin (CT) is a specific biomarker for medullary thyroid carcinoma (MTC).

**Why the Correct Answer is Right:**