Medicine – Biochemical Patterns in Metabolic Bone and Parathyroid Disease

The image compares the characteristic changes in serum calcium (Ca²⁺), phosphate (PO₄³⁻), alkaline phosphatase (ALP), and parathyroid hormone (PTH) in several important metabolic bone and endocrine disorders.

These patterns are extremely useful for distinguishing primary hyperparathyroidism, hypoparathyroidism, osteoporosis, osteomalacia, Paget disease, and chronic kidney disease–mineral and bone disorder.

1. Primary Hyperparathyroidism

Primary hyperparathyroidism is caused by autonomous excessive secretion of PTH from one or more parathyroid glands.

The most common cause is:

Parathyroid adenoma.

Other causes include:

Multigland parathyroid hyperplasia

and, rarely:

Parathyroid carcinoma.

Biochemical Pattern

Calcium: ↑

Phosphate: ↓ or normal

ALP: ↑ or normal

PTH: ↑

Why Calcium Increases

PTH raises serum calcium through several mechanisms.

It increases:

Renal calcium reabsorption.

It stimulates renal production of:

Calcitriol – 1,25-dihydroxyvitamin D.

Calcitriol then increases intestinal calcium absorption.

PTH also increases bone turnover, indirectly stimulating osteoclastic bone resorption through osteoblast-lineage signalling.

Therefore:

↑ PTH → ↑ serum Ca²⁺.

Why Phosphate Falls

PTH decreases phosphate reabsorption in the:

Proximal renal tubule.

This produces:

Phosphaturia – increased urinary phosphate excretion.

Therefore:

↑ PTH → ↑ urinary PO₄³⁻ loss → ↓ serum phosphate.

This gives the classic pattern:

HIGH CALCIUM + LOW PHOSPHATE + HIGH PTH.

Why ALP May Increase

ALP reflects:

Osteoblastic activity and bone turnover.

In mild primary hyperparathyroidism, ALP may remain:

Normal.

With more substantial skeletal involvement and increased bone turnover:

ALP rises.

Therefore:

ALP = normal or ↑.

2. Hypoparathyroidism

Hypoparathyroidism results from:

Deficient PTH secretion.

A common acquired cause is damage to or removal of the parathyroid glands during:

Neck or thyroid surgery.

Other causes include autoimmune disease and genetic disorders.

Biochemical Pattern

Calcium: ↓

Phosphate: ↑

ALP: usually normal

PTH: ↓

Why Calcium Falls

With inadequate PTH:

Renal calcium reabsorption decreases

and

Calcitriol production decreases.

This reduces intestinal calcium absorption.

Therefore:

↓ PTH → ↓ serum Ca²⁺.

Why Phosphate Increases

Normally PTH promotes urinary phosphate excretion.

Without PTH:

Renal phosphate reabsorption increases.

Therefore phosphate accumulates:

↓ PTH → ↓ phosphaturia → ↑ serum phosphate.

Classic Pattern

Think:

HYPOPARATHYROIDISM

↓

LOW PTH

↓

LOW CALCIUM

• ●

HIGH PHOSPHATE.

This is almost the biochemical mirror image of primary hyperparathyroidism.

3. Osteoporosis

Osteoporosis is characterised by reduced bone mass and deterioration of bone microarchitecture, resulting in increased:

Bone fragility and fracture risk.

The bone is reduced in quantity, but its mineralisation is generally:

Normal.

This distinction explains the laboratory findings.

Biochemical Pattern

Calcium: Normal

Phosphate: Normal

ALP: Normal

PTH: Normal

Why Are the Blood Tests Normal?

Osteoporosis is not primarily a failure of mineralisation.

Instead, there is:

Reduced amount of normally mineralised bone.

Therefore routine calcium metabolism blood tests are usually:

Normal.

This is a very important examination point.

Key Osteoporosis Pattern

FRACTURE + LOW BONE DENSITY

with:

NORMAL Ca²⁺

NORMAL PO₄³⁻

NORMAL ALP

NORMAL PTH

suggests:

OSTEOPOROSIS.

Abnormal results should prompt investigation for secondary metabolic bone disease.

4. Osteomalacia

Osteomalacia is defective mineralisation of newly formed osteoid in adults.

In children, defective mineralisation involving growing bones is called:

Rickets.

A major cause is:

Vitamin D deficiency.

Typical Biochemical Pattern in Vitamin D Deficiency

Calcium: ↓ or sometimes normal

Phosphate: ↓

ALP: ↑

PTH: ↑

The image simplifies calcium as:

↓.

However, calcium may remain within the normal range because secondary hyperparathyroidism helps maintain serum calcium.

5. Why Calcium Falls in Osteomalacia

Vitamin D normally promotes intestinal absorption of:

Calcium

and

Phosphate.

Vitamin D deficiency therefore causes:

↓ intestinal calcium absorption.

The fall in calcium stimulates:

PTH secretion.

Therefore secondary hyperparathyroidism develops.

6. Why Phosphate Falls in Osteomalacia

Increased PTH attempts to preserve serum calcium.

However, PTH simultaneously causes:

Renal phosphate wasting.

Therefore:

Vitamin D deficiency

↓

↓ Ca²⁺ absorption

↓

↑ PTH

↓

↑ urinary phosphate excretion

↓

↓ serum phosphate.

7. Why ALP Rises in Osteomalacia

Defective mineralisation stimulates increased osteoblastic activity.

Therefore:

Bone ALP rises.

A high ALP is a particularly useful clue when distinguishing osteomalacia from uncomplicated osteoporosis.

Osteomalacia Pattern

Think:

LOW/LOW-NORMAL Ca²⁺

• ●

LOW PO₄³⁻

• ●

HIGH ALP

• ●

HIGH PTH

=

VITAMIN D DEFICIENCY OSTEOMALACIA in the appropriate clinical context.

8. Osteoporosis Versus Osteomalacia

This is an important distinction.

OSTEOPOROSIS:

There is too little bone, but remaining bone is normally mineralised.

Therefore:

Ca²⁺ normal.

PO₄³⁻ normal.

ALP usually normal.

PTH usually normal.

OSTEOMALACIA:

There is defective mineralisation of osteoid.

In typical vitamin D deficiency:

Ca²⁺ low or low-normal.

PO₄³⁻ low.

ALP high.

PTH high.

Therefore:

NORMAL BIOCHEMISTRY → think osteoporosis.

HIGH ALP + SECONDARY HYPERPARATHYROIDISM → think osteomalacia, depending on the cause.

9. Paget Disease of Bone

Paget disease of bone is characterised by markedly increased and disorganised:

Bone remodelling.

There is initially increased osteoclastic resorption followed by excessive osteoblastic bone formation.

The resulting bone may become:

Enlarged but structurally abnormal.

Biochemical Pattern

Calcium: Normal

Phosphate: Normal

ALP: ↑↑

PTH: Normal

This is one of the most characteristic patterns in the image.

10. Why ALP Is Very High in Paget Disease

Paget disease produces marked:

Osteoblastic activity.

Bone-specific alkaline phosphatase therefore rises substantially.

However, systemic calcium and phosphate homeostasis usually remains intact.

Therefore:

Ca²⁺ = normal

and

PO₄³⁻ = normal.

Classic Paget Pattern

NORMAL Ca²⁺

• ●

NORMAL PO₄³⁻

• ●

MARKEDLY HIGH ALP

=

THINK PAGET DISEASE.

If total ALP is elevated, liver disease should also be considered; liver enzymes or bone-specific ALP can help identify the source.

11. Clinical Features of Paget Disease

Many patients are:

Asymptomatic.

When symptomatic, possible features include:

Bone pain.

Bone deformity.

Increasing head size from skull involvement.

Hearing impairment.

Pathological fractures.

Secondary osteoarthritis.

Rarely, malignant transformation to osteosarcoma can occur.

12. Renal Failure / Chronic Kidney Disease

The image uses the older broad term:

Renal failure.

For this biochemical pattern, the more precise context is usually:

Advanced chronic kidney disease – CKD, particularly CKD–mineral and bone disorder.

Typical Biochemical Pattern

Calcium: ↓ or sometimes normal

Phosphate: ↑

ALP: ↑ or normal

PTH: ↑

This reflects:

Secondary hyperparathyroidism due to CKD.

13. Why Phosphate Rises in CKD

As GFR declines, the kidneys become progressively less able to excrete:

Phosphate.

Therefore phosphate retention develops.

In advanced disease:

↓ renal phosphate excretion → ↑ serum phosphate.

14. Why Calcium Falls in CKD

Diseased kidneys have reduced ability to convert vitamin D into its active form:

Calcitriol – 1,25-dihydroxyvitamin D.

Therefore:

↓ Calcitriol

↓

↓ Intestinal calcium absorption

↓

Tendency toward:

Hypocalcaemia.

Phosphate retention also contributes to disturbances in calcium balance.

15. Secondary Hyperparathyroidism in CKD

The combination of:

Phosphate retention

• ●

Reduced calcitriol

• ●

Low or low-normal calcium

stimulates the parathyroid glands.

Therefore:

PTH rises.

This is:

Secondary hyperparathyroidism.

16. Why ALP May Increase

Persistent secondary hyperparathyroidism can produce high-turnover bone disease:

Osteitis fibrosa.

Increased bone turnover causes:

ALP ↑.

However, CKD bone disease is heterogeneous, and patients with low-turnover adynamic bone disease may not have elevated ALP.

Therefore the image correctly gives:

ALP ↑ or normal.

17. CKD Pattern

Think:

ADVANCED CKD

↓

Phosphate retention

• ●

↓ Calcitriol

↓

↓/normal Ca²⁺

• ●

↑ PO₄³⁻

↓

↑ PTH

↓

Secondary hyperparathyroidism.

Therefore:

LOW/LOW-NORMAL Ca²⁺ + HIGH PO₄³⁻ + HIGH PTH → THINK CKD-RELATED SECONDARY HYPERPARATHYROIDISM.

18. Primary Versus Secondary Hyperparathyroidism

These two patterns are worth separating carefully.

PRIMARY HYPERPARATHYROIDISM:

PTH is autonomously increased.

Therefore:

PTH ↑

Ca²⁺ ↑

PO₄³⁻ ↓ or normal

ALP normal or ↑.

SECONDARY HYPERPARATHYROIDISM DUE TO CKD:

PTH rises appropriately in response to abnormal mineral metabolism.

Therefore:

PTH ↑

Ca²⁺ ↓ or normal

PO₄³⁻ ↑ in advanced CKD

ALP normal or ↑.

19. Vitamin D Deficiency Versus CKD

Both can cause:

Secondary hyperparathyroidism.

Therefore both may have:

↑ PTH

and

↑ ALP.

The phosphate helps distinguish the classic patterns.

VITAMIN D DEFICIENCY OSTEOMALACIA:

PO₄³⁻ ↓

because secondary hyperparathyroidism increases renal phosphate excretion.

ADVANCED CKD:

PO₄³⁻ ↑

because the kidneys cannot adequately excrete phosphate.

Therefore:

HIGH PTH + LOW PHOSPHATE → think vitamin D deficiency.

HIGH PTH + HIGH PHOSPHATE → think advanced CKD.

20. Biochemical Patterns – Copyable Note Form

PRIMARY HYPERPARATHYROIDISM

Calcium:

↑

Phosphate:

↓ or normal

ALP:

↑ or normal

PTH:

↑

Classic clue:

High calcium + high PTH.

HYPOPARATHYROIDISM

Calcium:

↓

Phosphate:

↑

ALP:

Normal

PTH:

↓

Classic clue:

Low calcium + high phosphate + low PTH.

OSTEOPOROSIS

Calcium:

Normal

Phosphate:

Normal

ALP:

Normal

PTH:

Normal

Classic clue:

Metabolic bone blood tests are usually normal.

OSTEOMALACIA – TYPICAL VITAMIN D DEFICIENCY

Calcium:

↓ or low-normal

Phosphate:

↓

ALP:

↑

PTH:

↑

Classic clue:

High ALP + low phosphate + secondary hyperparathyroidism.

PAGET DISEASE

Calcium:

Normal

Phosphate:

Normal

ALP:

↑↑

PTH:

Normal

Classic clue:

Isolated marked elevation of ALP with normal calcium and phosphate.

ADVANCED CKD / CKD-MINERAL AND BONE DISORDER

Calcium:

↓ or normal

Phosphate:

↑

ALP:

↑ or normal

PTH:

↑

Classic clue:

High phosphate + high PTH.

21. Important Clarifications to the Image

The image is useful for examination pattern recognition, but real patients do not always fit every arrow exactly.

In primary hyperparathyroidism, phosphate may be:

Low or low-normal, and ALP may remain normal in mild disease.

In vitamin D deficiency osteomalacia, calcium does not have to be frankly low. Secondary hyperparathyroidism may maintain it within the:

Low-normal or normal range.

The more useful pattern is:

Low phosphate + high ALP + high PTH.

In CKD, calcium and phosphate abnormalities depend on the stage of kidney disease and treatment. Serum phosphate may remain normal until more advanced CKD.

Therefore:

High phosphate + high PTH is particularly characteristic of more advanced CKD-related mineral disturbance.

Key Clinical Pattern

For rapid recall:

PRIMARY HYPERPARATHYROIDISM

Ca ↑ | PO₄ ↓ | PTH ↑

HYPOPARATHYROIDISM

Ca ↓ | PO₄ ↑ | PTH ↓

OSTEOPOROSIS

Ca N | PO₄ N | ALP N | PTH N

OSTEOMALACIA

Ca ↓/N | PO₄ ↓ | ALP ↑ | PTH ↑

PAGET DISEASE

Ca N | PO₄ N | ALP ↑↑ | PTH N

ADVANCED CKD

Ca ↓/N | PO₄ ↑ | ALP ↑/N | PTH ↑

The fastest high-yield associations are:

HIGH Ca + HIGH PTH → PRIMARY HYPERPARATHYROIDISM.

LOW Ca + HIGH PO₄ + LOW PTH → HYPOPARATHYROIDISM.

NORMAL EVERYTHING → OSTEOPOROSIS.

LOW PO₄ + HIGH ALP + HIGH PTH → OSTEOMALACIA/VITAMIN D DEFICIENCY.

NORMAL Ca/PO₄ + VERY HIGH ALP → PAGET DISEASE.

HIGH PO₄ + HIGH PTH + LOW/LOW-NORMAL Ca → ADVANCED CKD WITH SECONDARY HYPERPARATHYROIDISM.