**Question:** Serum marker used after surgery for differentiated thyroid cancer is

A. Calcitonin
B. Carcinoembryonic antigen (CEA)
C. Thyroglobulin (Tg)
D. Thyroid-stimulating hormone (TSH)

**Core Concept:**

Thyroid cancer is a type of endocrine cancer that originates in the thyroid gland. Differentiated thyroid cancer refers to cancers that retain the functional differentiation of the thyroid gland, i.e., the ability to produce thyroid hormones (T3 and T4) and thyrotropin-releasing hormone (TRH). The primary treatment for thyroid cancer is surgery, aiming to remove the tumor and surrounding tissue. Post-surgery, monitoring of specific serum markers helps in assessing tumor recurrence or persistence.

**Why the Correct Answer is Right:**

Thyroid-stimulating hormone (TSH) is a hormone produced by the anterior pituitary gland that stimulates the thyroid gland to produce thyroid hormones (T3 and T4). In the context of differentiated thyroid cancer, TSH levels should decrease after successful surgery and treatment. However, TSH alone may not be specific enough to detect early recurrence, as it can be influenced by various factors such as thyroxine (T4) therapy and remnant thyroid tissue.

**Why Each Wrong Option is Incorrect:**

**A. Calcitonin**:
Calcitonin is a hormone secreted by C cells in the thyroid gland in response to high calcium levels. It is primarily used for monitoring medullary thyroid cancer (carcinoma of C cells), which is a rare type of thyroid cancer with a different pathogenesis than differentiated thyroid cancer. Calcitonin is not specific for differentiated thyroid cancer monitoring.

**B. Carcinoembryonic antigen (CEA)**:
CEA is a tumor marker elevated in various gastrointestinal and pancreatic tumors but not specifically in thyroid cancer. It is not a reliable marker for monitoring differentiated thyroid cancer.

**C. Thyroglobulin (Tg)**:
Thyroglobulin is a protein produced by the thyroid follicular cells and plays a crucial role in thyroid hormone synthesis. In the context of differentiated thyroid cancer, Tg levels are suppressed after surgery and radioactive iodine therapy, indicating successful treatment. Monitoring Tg levels helps detect early recurrence or persistence of the disease.

**D. Thyroid-stimulating hormone (TSH)**:
As mentioned earlier, TSH levels decrease after successful differentiated thyroid cancer treatment. However, it is not specific for thyroid cancer monitoring. Monitoring TSH levels can be influenced by thyroxine (T4) therapy and remnant thyroid tissue. Therefore, monitoring Tg along with TSH is more reliable for assessing differentiated thyroid cancer prognosis.

**Clinical Pearl:**
After total thyroidectomy, TSH levels should be suppressed in patients with differentiated thyroid cancer. If TSH levels rise, it may indicate disease recurrence or persistence, warranting further investigations like Tg measurement. However, monitoring TSH alone is insufficient. Monitoring Tg and TSH together provides a more reliable assessment of the disease status in thyroid cancer patients.