**Question:** A 45-year-old woman complains of tingling in her hands and feet, 24 hours after removal of follicular thyroid carcinoma. Her symptoms rapidly progress to severe muscle cramps, laryngeal stridor, and convulsions. Which of the following laboratory findings would be expected in this patient prior to treatment?

A. Increased serum calcium level
B. Increased serum potassium level
C. Increased serum creatine kinase (CK) level
D. Increased serum lactate level

**Correct Answer:** **C. Increased serum creatine kinase (CK) level**

**Core Concept:** Thyroid carcinoma can cause paraneoplastic syndromes, which are a group of conditions that occur due to the release of abnormal substances by the tumor or as a result of the tumor causing damage to surrounding tissues. In the case of thyroid carcinoma, one such paraneoplastic syndrome is the release of thyrotropin-releasing hormone (TRH) by the tumor. TRH stimulates the pituitary gland to produce thyrotropin (TSH), which in turn stimulates the thyroid gland to produce thyroxine (T4) and triiodothyronine (T3). This leads to an increase in serum calcium levels due to enhanced calcium release from bones and increased calcium reabsorption in the kidneys.

**Why the Correct Answer is Right:**

The correct answer (C) is related to the paraneoplastic syndrome caused by the release of thyrotropin-releasing hormone (TRH) by the thyroid carcinoma. This TRH stimulates the pituitary gland to produce thyrotropin (TSH), which leads to increased production of thyroxine (T4) and triiodothyronine (T3) by the thyroid gland.

Increased TSH levels cause the thyroid gland to produce more thyroid hormones, which results in a condition known as thyrotoxicosis (overactive thyroid). Thyrotoxicosis can present with symptoms like tingling in the hands and feet, muscle weakness, and severe muscle cramps.

**Why Other Options are Incorrect:**

Option A (increased serum calcium level) is incorrect because thyrotoxicosis primarily affects thyroid hormone levels, not calcium. While thyrotoxicosis can lead to increased calcium release from the bones, the primary cause of hypercalcemia (high calcium levels) in this case is the tumor's release of TRH, not the thyroid hormones themselves.

Option B (increased serum potassium level) is incorrect because thyrotoxicosis does not typically cause a significant increase in serum potassium levels. Hypokalemia (low potassium levels) is seen more commonly with hypothyroidism, not thyrotoxicosis.

Option D (increased serum lactate level) is incorrect as thyrotoxicosis does not lead to increased lactate production. While thyroid hormones can affect energy production and utilization in tissues, increased lactate levels are more commonly associated with hypoxia, sepsis, or other conditions that impair oxidative phosphorylation in mitochondria.

**Clinical Pearls:**

1. Thyrotoxicosis is a paraneoplastic syndrome caused by the release of TRH and subsequent