📚 Endocrinology — L8: Parathyroid Hormones & Disorders

📚 Lecture Overview

This lecture covers the physiology of parathyroid hormone (PTH) and vitamin D in calcium homeostasis, along with the pathophysiology, diagnosis, and management of parathyroid disorders. Mastering these concepts is essential for understanding how the body regulates serum calcium and how disruptions lead to clinical emergencies like hypercalcemia and hypocalcemia.

🎯 Key Concepts & Definitions

• Primary Hyperparathyroidism (PHPT): Autonomous, inappropriate overproduction of PTH from the parathyroid glands, typically caused by a solitary adenoma.
• Secondary Hyperparathyroidism: A compensatory increase in PTH secretion driven by chronic hypocalcemia, most commonly seen in chronic kidney disease (CKD) or vitamin D deficiency.
• Tertiary Hyperparathyroidism: Autonomous PTH secretion that persists long-term even after the original secondary stimulus (such as renal failure) is corrected.
• Hypoparathyroidism: A state of deficient or ineffective PTH leading to hypocalcemia, most frequently caused by accidental surgical removal or damage during anterior neck surgery.
• Pseudohypoparathyroidism (PHP): A genetic disorder characterized by end-organ resistance to PTH, resulting in hypocalcemia, hyperphosphatemia, and elevated PTH levels.
• Familial Hypocalciuric Hypercalcemia (FHH): A benign autosomal dominant condition caused by a mutation in the calcium-sensing receptor (CaSR) leading to lifelong mild hypercalcemia and low urinary calcium excretion.

Main Content

Anatomy and Histology of the Parathyroids

• Typically four glands located on the posterior aspect of the thyroid gland, originating from the 3rd and 4th pharyngeal pouches.
• Composed of two main cell types:
• Chief Cells: Most abundant; synthesize and secrete Parathyroid Hormone (PTH).
• Oxyphil Cells: Larger cells with eosinophilic cytoplasm packed with mitochondria; increase in number with age.
• Normal size is lentil-sized (3-6 mm, weighing 30-50 mg).

Physiology of PTH and Vitamin D

• PTH Regulation: Secretion is stimulated by low ionized serum calcium detected by the Calcium-Sensing Receptor (CaSR) on chief cells.
• Major Actions of PTH:
• Bone: Stimulates osteoclast-mediated bone resorption to release calcium and phosphate.
• Kidney: Increases calcium reabsorption in the distal tubule, decreases phosphate reabsorption (phosphaturia) in the proximal tubule, and stimulates 1-alpha-hydroxylase to activate vitamin D.
• Vitamin D Activation:
• Converted in the liver to 25-Hydroxyvitamin D [25(OH)D] (best indicator of total body stores).
• Converted in the kidney by 1-alpha-hydroxylase to 1,25-Dihydroxyvitamin D (Calcitriol), the active form.

Hyperparathyroidism and Hypercalcemia

• Causes of Hypercalcemia: PHPT and Malignancy account for roughly 90% of cases.
• Classic Mnemonic ("Stones, Bones, Groans, and Psychiatric Overtones"):
• Stones: Nephrolithiasis, polyuria, polydipsia, renal impairment.
• Bones: Osteitis fibrosa cystica (subperiosteal resorption, brown tumors), bone pain, osteopenia.
• Groans: Anorexia, nausea, vomiting, constipation, peptic ulcer disease, pancreatitis.
• Psychiatric Overtones: Fatigue, depression, anxiety, cognitive dysfunction, lethargy, coma.
• Diagnostic Compass:
• High PTH + High Calcium = Primary HPT
• High PTH + Low Calcium = Secondary HPT
• Low PTH + High Calcium = Malignancy / Non-Parathyroid cause
• Low PTH + Low Calcium = Hypoparathyroidism
• Management of PHPT:
• Surgical Parathyroidectomy is the only cure, indicated for symptomatic disease or specific guideline cutoffs (e.g., calcium >1 mg/dL above normal, eGFR <60, osteoporosis).
• Conservative Surveillance: Hydration, normal dietary calcium, avoiding thiazide diuretics and immobilization, monitoring calcium, renal function, and DXA scans.

Hypoparathyroidism and Hypocalcemia

• Key Causes: Post-surgical neck operations (most common), autoimmune destruction (APS-1), DiGeorge syndrome, and magnesium deficiency.
• Clinical Manifestations (Neuromuscular Irritability):
• Paresthesias, muscle cramps, carpopedal spasm, laryngospasm, and seizures.
• Chvostek's Sign: Facial muscle twitching when tapping the facial nerve anterior to the ear.
• Trousseau's Sign: Carpal spasm induced by inflating a blood pressure cuff above systolic pressure for 3 minutes.
• ECG finding: Prolonged QT interval (risk of torsades de pointes).
• Treatment Strategy:
• Oral elemental calcium (1,000–3,000 mg/day) and active vitamin D (Calcitriol).
• Goal: Maintain serum calcium at the low-normal range (8.0–8.6 mg/dL) while avoiding hypercalciuria and nephrocalcinosis.

📊 Visual Learning

💡 Important Points to Remember

• PTH and Phosphate: PTH is unique because while it raises serum calcium, it lowers serum phosphate via renal wasting (phosphaturia).
• The Best Status Test: 25-hydroxyvitamin D [25(OH)D] is the best marker for overall vitamin D stores, whereas Calcitriol is the active hormone.
• FHH Triad: Familial Hypocalciuric Hypercalcemia features mild hypercalcemia, normal/high PTH, and low urine calcium excretion (do not perform surgery!).
• DXA Scanning Sites: In PHPT, bone loss predominantly affects cortical bone; therefore, scanning must include the distal 1/3 radius alongside the spine and hip.
• Chvostek vs. Trousseau: Trousseau's sign is more specific for hypocalcemia than Chvostek's sign (which can be positive in 10-25% of normal individuals).
• Magnesium Link: Hypomagnesemia impairs both PTH secretion and action, mimicking hypoparathyroidism and rendering hypocalcemia refractory until magnesium is repleted.
• Renal Failure Paradox: CKD causes secondary hyperparathyroidism because failing kidneys cannot activate vitamin D or excrete phosphate, dragging calcium down.
• Treatment Trap in Hypoparathyroidism: Over-treating hypocalcemia with calcium and active vitamin D frequently causes hypercalciuria and nephrocalcinosis; urine calcium must be closely monitored.
• Calcium Formulations: Calcium carbonate requires an acidic gastric environment and should be taken with meals, whereas calcium citrate is preferred for patients on PPIs.

⚠️ Common Exam Questions

• Distinguishing Causes of Hypercalcemia: Examiners love testing the differential of high calcium. A high calcium with a high or normal PTH points straight to PHPT, whereas a high calcium with a suppressed/low PTH points to malignancy (secreting PTHrP).
• The FHH Trick: Questions will present an asymptomatic patient with mild hypercalcemia and normal PTH, making students rush to choose surgery. The trap is missed by ignoring the low 24-hour urine calcium, which confirms benign FHH.
• Pseudohypoparathyroidism Trap: Students see high PTH and hypocalcemia and assume secondary HPT. However, if the patient presents with short stature, round face, and shortened metacarpals (Albright's Hereditary Osteodystrophy), it is end-organ resistance (Pseudohypoparathyroidism).
• ECG Distinctions: Remember that hypercalcemia causes a shortened QT interval, while hypocalcemia causes a prolonged QT interval.

📝 Quick Review Checklist

I can explain the physiological actions of PTH on bone, kidney, and gut
I understand the multi-step activation pathway of Vitamin D in the liver and kidney
I can define and differentiate primary, secondary, and tertiary hyperparathyroidism
I know the classic clinical features and mnemonic for hypercalcemia (Stones, Bones, Groans, Psychiatric Overtones)
I can interpret fundamental lab panels (PTH, Calcium, Phosphate, Vit D) to diagnose parathyroid disorders
I recognize the signs of neuromuscular irritability in hypocalcemia (Trousseau's and Chvostek's signs)
I know the surgical and medical management principles for PHPT and Hypoparathyroidism