USMLE Guide: Parathyroid Disorders

Introduction

Parathyroid disorders refer to a group of conditions that affect the function of the parathyroid glands. These small glands, located in the neck, play a vital role in regulating calcium and phosphate levels in the body. An understanding of parathyroid disorders is crucial for medical professionals preparing for the USMLE exams. This guide will provide an overview of key concepts, clinical presentations, diagnostic methods, and treatment options for parathyroid disorders.

Key Concepts

1. Parathyroid Hormone (PTH)

Parathyroid hormone is the primary hormone secreted by the parathyroid glands. It functions to maintain calcium homeostasis by increasing serum calcium levels through various mechanisms, such as promoting bone resorption and enhancing renal reabsorption of calcium.

2. Hyperparathyroidism

Hyperparathyroidism is characterized by excessive secretion of parathyroid hormone, resulting in hypercalcemia. It can be classified into primary, secondary, and tertiary hyperparathyroidism.

• Primary hyperparathyroidism: Caused by a benign tumor (adenoma) of the parathyroid gland, leading to increased PTH secretion.
• Secondary hyperparathyroidism: Occurs as a compensatory response to hypocalcemia, often due to chronic kidney disease or vitamin D deficiency.
• Tertiary hyperparathyroidism: A complication of long-standing secondary hyperparathyroidism, where the parathyroid glands become autonomous and continue to secrete PTH even after correction of the underlying cause.

3. Hypoparathyroidism

Hypoparathyroidism refers to the insufficient production or secretion of parathyroid hormone, leading to hypocalcemia. It can be inherited (genetic) or acquired (e.g., post-surgical removal of parathyroid glands). Common causes include autoimmune destruction, DiGeorge syndrome, and surgical damage during thyroidectomy.

4. Clinical Presentations

The clinical presentations of parathyroid disorders can vary depending on the degree of calcium imbalance and the chronicity of the disease. Key features include:

• Hyperparathyroidism: Asymptomatic hypercalcemia, kidney stones, bone pain/fractures, abdominal pain, constipation, polyuria, and neuropsychiatric symptoms.
• Hypoparathyroidism: Tetany (muscle cramps, spasms, paresthesias), Chvostek's sign (facial muscle spasm with tapping of the facial nerve), Trousseau's sign (carpal spasm with blood pressure cuff inflation).

Diagnostic Methods

1. Laboratory Tests

• Serum calcium levels: Elevated in hyperparathyroidism and decreased in hypoparathyroidism.
• Parathyroid hormone (PTH) levels: Elevated in primary and tertiary hyperparathyroidism, decreased in hypoparathyroidism.
• Serum phosphate levels: Decreased in hyperparathyroidism and increased in hypoparathyroidism.

2. Imaging Studies

• Ultrasonography: Helpful for detecting parathyroid adenomas.
• Technetium-99m sestamibi scan: A nuclear medicine study that localizes overactive parathyroid glands.

3. Genetic Testing

• Genetic testing may be indicated in cases of suspected familial hypoparathyroidism.

Treatment Options

1. Hyperparathyroidism

• Surgical removal (parathyroidectomy) is the definitive treatment for primary hyperparathyroidism.
• Close monitoring of calcium levels and conservative management may be appropriate for mild or asymptomatic cases.

2. Hypoparathyroidism

• Oral calcium and active vitamin D supplementation to maintain serum calcium levels.
• Long-term management includes regular monitoring of calcium, phosphate, and vitamin D levels.

Conclusion

Parathyroid disorders encompass a spectrum of conditions characterized by abnormal parathyroid gland function and calcium imbalance. Understanding the key concepts, clinical presentations, diagnostic methods, and treatment options is essential for medical professionals preparing for the USMLE exams. Regular review and practice of these concepts will help ensure success on exam day.