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PA31.{4,6} | Thyroid Tumors & Parathyroid Disease — SDL Guide (Part 3)

Thyroid Carcinoma Comparison: At a Glance

This table consolidates the four major thyroid carcinomas for examination revision:

Feature	Papillary (PTC)	Follicular (FTC)	Medullary (MTC)	Anaplastic
Frequency	80–85%	10–15%	5–10%	1–2%
Cell of origin	Follicular	Follicular	C cell (parafollicular)	Follicular (dedifferentiated)
Age	20–50	40–60	Variable	>65
Key molecular	BRAF V600E, RET/PTC	RAS, PAX8-PPARG	Germline/somatic RET	TP53, BRAF
Histologic hallmark	Orphan-Annie nuclei, psammoma bodies	Capsular/vascular invasion	Amyloid stroma	Pleomorphic giant cells
Spread	Lymphatic (cervical nodes)	Hematogenous (lung, bone)	Both	Direct invasion + hematogenous
Serum marker	Thyroglobulin (post-op)	Thyroglobulin (post-op)	Calcitonin, CEA	None useful
Radioiodine response	Yes (especially differentiated)	Yes	No	No
Prognosis	Excellent	Good (minimally invasive) to moderate (widely invasive)	Intermediate	Uniformly fatal

Memory anchor for spread patterns: "Papillary goes to Lymph nodes; Follicular goes to the Far field (bones/lungs)"

Four-panel H&E histology comparison of thyroid carcinomas: Panel A shows papillary carcinoma with Orphan-Annie nuclei and a psammoma body; Panel B shows follicular carcinoma with capsular invasion; Panel C shows medullary carcinoma with amyloid stroma and plasmacytoid cells; Panel D shows anaplastic carcinoma with pleomorphic giant cells and necrosis. — **Panel A:** Orphan-Annie (ground-glass) nuclei with nuclear grooves; psammoma body (concentric calcified lamella); papillary frond architecture. **Panel B:** Microfollicular pattern; thick fibrous capsule; site of capsular invasion by tumor cells. **Panel C:** Plasmacytoid and spindle-shaped neuroendocrine cells; amorphous pale-pink amyloid stroma between cell nests. **Panel D:** Pleomorphic giant cells with multiple bizarre nuclei; mitotic figures; background tumour necrosis.

Primary Hyperparathyroidism: Pathogenesis and Causes

Hyperparathyroidism is defined as excess secretion of parathyroid hormone (PTH), leading to hypercalcemia and hypophosphatemia.

Classification:

Type	Mechanism	PTH	Calcium	Cause
Primary	Autonomous PTH secretion	↑↑	↑↑	Parathyroid adenoma (85%), hyperplasia (14%), carcinoma (1%)
Secondary	Appropriate PTH ↑ in response to chronic hypocalcemia	↑↑	Low or normal	CKD (most common), malabsorption, vitamin D deficiency
Tertiary	Secondary → autonomous hyperfunction despite resolved stimulus	↑↑↑	↑↑	Long-standing secondary hyperparathyroidism (CKD, post-transplant)

Primary Hyperparathyroidism — Causes in Detail:

Parathyroid adenoma (85%) — benign neoplasm of a single gland (usually inferior parathyroid). Characteristic rim of normal parathyroid tissue at the periphery (compressed normal gland). Composed predominantly of chief cells (principal cells). No evidence of multiple gland involvement.

Primary chief cell hyperplasia (14%) — involves all four glands (multiglandular). Associated with MEN1 (MENIN gene, chromosome 11q13) and MEN2A (RET mutation). Asymmetric enlargement of all four glands.

Parathyroid carcinoma (<1%) — rare, but should be suspected with very high PTH, palpable neck mass, and severe hypercalcemia. Fibrous bands, perineural invasion, and vascular invasion on histology.

Genetics:
- MEN1 (Werner syndrome): Parathyroid hyperplasia + anterior pituitary tumor + pancreatic islet tumor (MENIN tumor suppressor gene)
- MEN2A (Sipple syndrome): MTC + pheochromocytoma + parathyroid hyperplasia (RET proto-oncogene)

Three-panel illustration showing: (A) gross specimen of a parathyroid adenoma — oval, reddish-brown, with a labelled rim of compressed normal yellow-tan parathyroid tissue; (B) size comparison between a normal parathyroid gland (<0.04 g) and an adenoma (0.5–5 g); (C) anatomical location of the adenoma on the posterior thyroid surface among all four parathyroid glands. — **Panel A:** Parathyroid adenoma (reddish-brown central mass), rim of normal parathyroid tissue (yellow-tan peripheral rim), fibrous capsule (demarcation line), 1 cm scale bar, millimeter ruler. **Panel B:** Normal parathyroid gland <0.04 g (lentil-sized, yellow-tan), parathyroid adenoma 0.5–5 g (large oval, reddish-brown), bidirectional mass-increase arrow (10–125× annotation). **Panel C:** Superior parathyroids ×2 (normal, yellow-tan), inferior parathyroids ×2 (one normal, one enlarged adenoma reddish-brown), posterior thyroid lobes (grey-blue schematic), adenoma location label with arrow.

Hyperparathyroidism: Clinical Manifestations and Lab Findings

The clinical manifestations of hyperparathyroidism are best remembered by the classic mnemonic:

"Stones, Bones, Groans, and Psychic Moans"

Component	Manifestation	Mechanism
Stones	Renal stones (nephrolithiasis), nephrocalcinosis	Hypercalciuria → calcium oxalate/phosphate stones
Bones	Bone pain, pathological fractures, osteitis fibrosa cystica	PTH → osteoclast activation → bone resorption
Groans	Nausea, vomiting, constipation, peptic ulcer (via gastrin)	Hypercalcemia → smooth muscle relaxation, gastrin secretion
Psychic moans	Depression, anxiety, cognitive impairment, confusion	Hypercalcemia → CNS depression, fatigue

Additional manifestations:
• Band keratopathy — calcium deposits at lateral corneal margins (slit-lamp finding)
• Hypertension — in ~50% of patients
• Shortened QT interval on ECG — classic electrocardiographic sign of hypercalcemia
• Polyuria/polydipsia — nephrogenic diabetes insipidus (hypercalcemia inhibits ADH action on renal tubule)

Laboratory findings — Primary Hyperparathyroidism:

Test	Finding	Reasoning
Serum calcium	↑↑	PTH releases bone calcium, increases renal reabsorption
Serum phosphate	↓	PTH promotes renal phosphate wasting
Serum PTH (intact)	↑ or inappropriately normal	Autonomous secretion (normal PTH in hypercalcemia = inappropriate = abnormal)
Urine calcium	↑	Overwhelms renal reabsorption
Alkaline phosphatase	↑	Osteoblastic activity secondary to bone turnover
1,25-(OH)₂ vitamin D	↑	PTH activates 1-α-hydroxylase in kidney

Key distinction: In malignancy-associated hypercalcemia, PTH-related protein (PTHrP) is elevated but intact PTH is suppressed (because the pituitary-parathyroid feedback axis is intact and responds to hypercalcemia).

Two-panel diagram: Panel A shows the PTH-calcium axis with the parathyroid gland releasing PTH to three target organs — bone (osteoclast-mediated Ca²⁺ release), kidney (Ca²⁺ reabsorption, phosphate excretion, 1,25-D3 activation), and intestine (Ca²⁺ absorption via 1,25-D3), converging to net ↑Ca²⁺ and ↓PO₄³⁻; Panel B shows a comparison table distinguishing primary hyperparathyroidism from humoral hypercalcemia of malignancy by PTH, PTHrP, and ALP levels. — **Panel A:** Parathyroid gland (source), PTH arrow (signal), Bone with osteoclast activation and Ca²⁺ release label, Kidney with Ca²⁺ reabsorption / PO₄³⁻ excretion / 1,25-D3 activation labels, Intestine with Ca²⁺ absorption via 1,25-D3 label, Net Result box (↑Ca²⁺ in red, ↓PO₄³⁻ in blue). **Panel B:** Comparison table headers (Marker / Primary HPT / HHM-Malignancy), PTH row, PTHrP row, ALP row, diagnostic note for unsuppressed PTH + hypercalcemia = Primary HPT.

CLINICAL PEARL

"Asymptomatic" primary hyperparathyroidism is common — and often missed. Today, >80% of primary HPT patients in developed countries are diagnosed incidentally on routine biochemistry before developing stones or bone disease. The finding of hypercalcemia with an unsuppressed (or elevated) PTH is sufficient to diagnose primary hyperparathyroidism — no other test is needed for the biochemical diagnosis.

Urgent distinction — PTH vs. PTHrP:
In an inpatient with hypercalcemia, the first question is: primary HPT or malignancy?
- Primary HPT: PTH ↑, PTHrP normal
- Malignancy (solid tumor): PTH suppressed ↓, PTHrP ↑ (humoral hypercalcemia of malignancy)
- Bone mets: PTH suppressed, PTHrP variable, alkaline phosphatase often very high
This single biochemical distinction guides the entire workup.