4-Parathyroid Hormone and Mineral Metabolism

Bone Tissue Composition

30% Organic

• Cells 2%

  • Osteoblasts (form bone)

  • Osteocytes (encased within bone, signaling)

  • Osteoclasts (macrophages, bone reabsorption)

• Matrix 98%

  • Collagen Type I

  • Proteins

70% Minerals

• Ca2+

• Phosphate

Type I Collagen

Crosslinks with itself to create a regularly reoccurring pattern that we see in bone, giving bone its strength.

Disrupting this pattern weakens bone.

Bone Degradation

This is when the type I collagen crosslinks can be released.

Bone Turnover Rate

Take an assay of N-telopeptides and C-telopeptides to get an idea of THIS.

Osteoblasts

Bone-forming cells that contain receptors for parathyroid hormone (PTH) and initiate the bone remodeling process.

PTH

A hormone that signals osteoblasts to start the bone remodeling process.

M-CSF (Monocyte Colony-Stimulating Factor)

A factor released by osteoblasts that helps recruit osteoclast precursors (STEM cells).

Vitamin D in Bone Remodeling

Works alongside M-CSF to promote osteoclast precursor differentiation.

Osteoclast Precursors

Immature bone-resorbing cells that develop from hematopoietic stem cells.

Hematopoietic Stem Cells

The type of stem cells that differentiate into osteoclast precursors (not mesenchymal stem cells).

Multinucleated Osteoclasts

Fully matured osteoclasts with a ruffled border, responsible for bone resorption.

IL-6 (Interleukin-6)

A signaling molecule that helps activate multinucleated osteoclasts.

RANK Ligand

A key signaling factor that promotes osteoclast differentiation and activation.

Bone Resorption

The process by which osteoclasts break down bone, releasing minerals and collagen fragments.

Enzymes Released by Osteoclasts

Chemical mediators that break down bone matrix and dissolve minerals.

Osteoblasts’ Role in Bone Remodeling

Follow osteoclasts and lay down new organic matrix, which later mineralizes into new bone.

Bone Matrix

The organic structure deposited by osteoblasts, which becomes mineralized to form new bone.

Osteocytes

Mature bone cells embedded in bone that monitor the environment and regulate bone formation and resorption.

Bone Remodeling Process Steps

1. PTH signals osteoblasts to release M-CSF and vitamin D.

2. M-CSF and vitamin D recruit hematopoietic stem cells to become osteoclast precursors.

3. Osteoclast precursors mature into multinucleated osteoclasts with a ruffled border.

4. Activated osteoclasts break down bone, releasing minerals and collagen fragments.

5. Osteoblasts follow osteoclasts, laying down a new organic matrix that will mineralize into bone.

6. Osteocytes, embedded within the bone, monitor the environment and regulate both bone formation and resorption.

Osteoprotegerin (OPG)

Released by osteoblasts

Acts as a soluble receptor for RANKL

Shuts down resorption by osteoclasts in bone-forming process

No RANKL → ↑ bone density (osteopetrosis: no activity of osteoclasts)

No OPG → ↓ bone density (osteoporosis: RANKL is always around/on)

Hormones Regulating Mineral Metabolism (what are they? what do they act on?)

Parathyroid hormone (PTH), vitamin D, calcitonin, and fibroblast growth factor (FGF23)

• Regulates levels of calcium and phosphate in the blood.

They act on cells in the intestine, kidney, bone, and parathyroid gland

Ca and Phosphate

Both THESE have concentrations within the blood, but some of that is Ionized (50%, 84%), Protein Bound (45%, 10%), and Complexed (5%, 6%).

Hypocalcemic

< 8.5 mg/dL (2.1 mM)

Sx:

• twitching

• muscle cramps

• tingling

• numbness

Hypercalcemic

> 10.5 mg/dL (2.6 mM)

Sx:

• constipation

• polyuria

• polydipsia

• lethargy

• coma

• death

Calcium and Parathyroid Hormone Connection

Calcium and parathyroid hormone (PTH)

• tightly connected

• regulate each other

Steps in the Regulation of Parathyroid Hormone Expression

1. Calcium binds to the calcium-sensing receptor, suppressing PTH expression (inhibition).

2. Vitamin D (active form) also suppresses PTH expression (inhibition).

3. When calcium levels drop below 9, PTH expression increases.

4. Calcium dissociates from the receptor, removing the inhibition.

5. PTH mRNA is expressed, leading to the formation of the preprohormone→prohormone, which is cleaved into its active form and released.

6. Only ionized calcium binds to the calcium-sensing receptor, regulating PTH expression.

CaSR

Calcium-Sensing Receptor

Ionized Ca binds this!

Key Functions of PTH

• Triggers Ca2+, PO43- resorption from bone (increases Ca levels)

• Promotes Ca2+ resorption from the kidney (increases Ca levels)

• Promotes PO43- excretion from the kidney (minimizing calcification risk)

• Signals 1alpha-hydroxylation of 25-hydroxycholealciferol in the kidney (increases Ca levels)

PTHrP (parathyroid hormone-related protein)

• Can be made by many tissues

• Has PTH-like actions

• Associated with humoral hypercalcemia of malignancy

• Locally expressed by tumors/lesions in the bone

• Acts as calciotropic hormone during fetal period/lactation

• Regulates cell proliferations/differentiation as an adult

PTH and Vitamin D

THESE are the 2 hormones associated with calcium regulation

Steps in Vitamin D Synthesis and Regulation

1. 7-dehydrocholesterol (cholesterol derivative) is exposed to UV light in the skin and converted to cholecalciferol (vitamin D3), can also come from the diet.

2. In the liver, cholecalciferol → 25-hydroxycholecalciferol (calcidiol).

   • In the kidney, 25-hydroxycholecalciferol is converted to:

3. Active form of Vit D (calcitriol) via 1-alpha hydroxylase

   1. 1,25 (OH)2- cholecalciferol

      1. Upregulated by low calcium & high PTH & low phosphate

4. Inactive form of Vit D via 24-hydroxylase.

   1. 24, 25-hydroxycholecalciferol (calcidiol)

1-alpha hydroxylase and 24-hydroxylase are used in blood tests to assess Vit D levels!

1,25-(OH)2-Vitamin D

• Promotes Ca2+ absorption from gut

• Shuts off its own synthesis (neg feedback)

• Promotes bone remodeling (promotes mineralization through increasing the absorption of Ca and Pi and bone resorption)

• Promotes renal resorption of Ca2+ and PO43- (bone remodeling)

Note: PTH promotes conversion of 25-OH-Vit D to 1,25(OH)2-Vit D

Absolutely NEED to have for bone formation!

Calcitonin

• Regulates calcium homeostasis (uncertain in humans tho)

• Made by non-follicular C cells in medullary thyroid cancer

  • Cancer marker!

• Lowers blood Ca levels in experiments

• Used to treat osteoporosis (salmon-derived) in the past (limited bone resorption)

FGF23 (fibroblast growth factor 23)

• Involved in phosphate homeostasis

• Promotes phosphate excretion

• Increased expression results in hypophosphatemia and impaired 1α-hydroxylation

• Expressed by osteocytes

• Clinical implications in the diagnosis and management of chronic kidney disease as well as some bone diseases

Inhibits Vit D and PTH, promotes loss of phosphate (causes phosphaturia when over-expressed)!

Additional hormone/growth factor regulators of bone homeostasis

• Systemic hormones (e.g. GH, T3 and T4, insulin) increase bone formation

• Glucocorticoids suppress bone formation

• Gonadal hormones (estrogen and androgens) increase bone formation

• Local regulators (e.g. cytokines related to the TNF subfamily) suppress bone formation

• Growth factors (e.g. IGF, PDGF, etc.) stimulate bone formation

Sclerostin

Additional protein involved in bone remodeling

Made by osteocytes

Anti-anabolic effects on bone formation

Regulates bone growth, prevents overgrowth

• Reduces bone mineralization (osteoblasts)

Target in treating osteoporosis!

Metabolic Bone Disease Types

• Osteoporosis

• Osteomalacia & Rickets

• Paget’s Disease

• Osteogenesis Imperfecta

Osteoporosis

Characterized by increased bone fragility and risk of fracture

Risk factors fall into nonmodifiable and modifiable categories

Osteomalacia and Rickets

Disorders in mineralization of bone and most often associated with vitamin D deficiency

Paget’s disease

Characterized by excessive osteoblast activity and hyperactive bone remodeling

Osteogenesis Imperfecta

Condition characterized by weaking of bone due to genetic mutations in collagen

Markers for Bone Disease

• Bone Densitometry

• Biochemical measurements (formation/reabsorption)

• Bone bx and imaging

Bone densitometry

T and Z scores can be obtained

T scores can be used to diagnose osteoporosis (<2.5 standard deviations is the criteria for diagnosis of osteoporosis)

Bone Formation Biochemical Measurements

Alkaline phosphatase, osteocalcin, N-terminal and C-terminal peptides of procollagen (PINP or PICP)

Bone Reabsorption Biochemical Measurements

Urinary hydroxyproline and collagen cross-links (NTX or N-telopeptides and CTX or C-telopeptides)

Parathyroid-Related Conditions

Primary hyperparathyroidism

Hypercalcemia of malignancy

Hypoparathyroidism

Primary hyperparathyroidism

Characterized by unregulated secretion of parathyroid hormone from the parathyroid gland (too much PTH)

Hypercalcemia of malignancy

Associated with metastatic cancer and driven by PTHrP or local inflammatory mediators (too little PTH due to excess Ca)

Increases bone resoprtion (Ca increased), decreases PTH

Hypoparathyroidism

Hypocalcemia driven by loss of parathyroid hormone production- either gland failure or gland removal (loss of PTH)

Bone Changes

Specifically seen in hyperparathyroidism!

PTH is secreted unregulated, too much PTH, driving bone resorption!

Decreased bone density!

Kidney Failure

Low blood Ca

High PTH

Issues with resorption, filtration, and excretion. Mainly no activation of Vit D! Causes low Ca, in response PTH rises!

Vitamin D Deficiency

Low blood Ca

Semi-High PTH

Need vit d for Ca! Not enough vit d means not enough Ca resorption!

Normal Ranges

Ca 9-10 mg/dL

PTH 20-40

Primary Hyperparathyroidism

High blood Ca

High PTH

Too much PTH → too much bone resorption → too much blood Ca

High Calcium of Malignancy

Very High Ca (caused by malignancy)

Low/No PTH

Hypercalcemic Disorders

• PTH Dependent

• PTH Independent

PTH Dependent Hypercalcemic Disorders

• Primary hyperparathyroidism (increased PTH)

• Familial hypocalciuric hypercalcemia

  • Heterozygous inactivating mutation in the CaSR (Ca sensory receptor)

  • Increased PTH and serum calcium

  • Low urine calcium

PTH Independent Hypercalcemic Disorders

• Hypercalcemia of malignancy (not due to PTH)

• Thiazide diuretics

• Immobilization

• Acute renal failure

• Granulomatous disease

Serum PTH

THIS is key test in the differential diagnosis of hypercalcemia.

Causes of Hypocalcemia

• Vitamin D Deficiency

• Chronic Renal Failure

• Liver Failure

• Hypoparathyroidism

• Pseudohypoparathyroidism

  • Genetic disorder causing resistance to PTH

C. Elevated PTH levels

A heterozygous inactivating mutation in the CaSR results in:

a. Hypercalciuria

b. Low levels of 1,25-dihydroxyvitamin D

c. Elevated PTH levels

d. Hypophosphatemia

C. Inhibiting secretion of SOST (sclerostin) by osteocytes

Intermittent treatment with PTH improves bone density through:

a. Stimulation of RANKL by osteoblasts

b. Induction of cell death by osteocytes

c. Inhibiting secretion of SOST (sclerostin) by osteocytes

d. Decreasing osteoprotegerin production by osteoblasts

B. Increase intestinal Ca2+ absorption

The primary action of 1,25-dihydroxyvitamin D to increase serum Ca2+ is to:

a. Stimulate osteoblast RANKL production

b. Increase intestinal Ca2+ absorption

c. Increase renal phosphate excretion

d. Stimulation PTH production

A. PTH 5 pg/mL, phosphate 6 mg/dL, alkaline phosphatase 600 U/L

 A 73-year-old woman is admitted to the hospital following a bout of severe vomiting and generalized weakness. Initial laboratory values reveal elevated Ca2+levels. The referring physician tells you that she has breast cancer, and her bone scan indicates metastasis to bone. Which of the following laboratory values would be compatible with this clinical scenario?

a. PTH 5 pg/mL, phosphate 6 mg/dL, alkaline phosphatase 600 U/L

b. PTH 90 pg/mL, phosphate 6 mg/dL, alkaline phosphatase 30 U/L

c. PTH 5 pg/mL, phosphate 2 mg/dL, alkaline phosphatase 20 U/L

d. PTH 3 pg/mL, phosphate 2 mg/dL, alkaline phosphatase 100 U/L

Note:

PTH (nl, 10-65 pg/ml))

Ca (nl, 8.5-10.5 mg/dL)

Phosphate (nl, 3.0-4.5 mg/dL)

Alkaline phosphate (nl, 25-125 U/L)

25- OH Vitamin D and PTH

Measurement of THESE are the key tests in the differential diagnosis of hypocalcemia.

Low

High

_ urine Ca would indicate familial form of hypoparathyroidism

_ urine Ca would indicate primary form of hypoparathyroidism

B. ionized Ca

Carl has unregulated secretion of PTH due to an adenoma of the parathyroid gland. Under physiologic control, which form of calcium stimulates PTH secretion?

a. Total calcium

b. Ionized calcium

c. Protein-bound calcium

d. Anion-complexed calcium

Overall readings of Ca

What can you conclude about normal Ca levels if albumin is elevated?

Albumin binds Ca. If Albumin levels start to change, it can change _.

Forms of Ca in the blood:

40% is protein bound (to Alubumin!)

60% is ultrafilterable (50% of this is Ionized Ca, the other 10% is complexed to anions)

A. osteoblast cells

In patients with mild hypercalcemia, observation may be appropriate for treatment. Observation should include monitoring blood calcium and creatinine every 6 months and yearly bone densitometry. On which cell type do the PTH receptors reside?

a. Osteoblast cells

b. Osteoclast cells

c. Osteocyte cells

d. Chrondrocytes

B. net bone resorption

What is the outcome of increased PTH secretion on the bone homeostasis?

a. Net bone deposition

b. Net bone resorption

c. No net change

Increased osteoblast activity due to increased PTH (increased M-CSF) and increased RANKL (both of which increase bone resorption)

What causes increased alkaline phosphatase levels in hyperparathyroidism?

• PTH signals to osteoblasts to release all STEM cell factors (M-CSF) to recruit osteoclasts.

• PTH also stimulates RANKL production.

• These both signal bone resorption! So, bone resorption is increased, but why is alkaline phosphatase?

  • Alkaline phosphatase is produced by osteoblasts and gives us an idea of what’s happening with bone formation.

  • So, bone resorption and bone formation are increased and linked together.

B. Vit D Deficiency

What is driving Ralph’s hypocalcemia?

a. Lack of calcium intake

b. Vitamin D deficiency

c. Phosphorus deficiency

d. Increased alkaline phosphatase levels

e. Elevated PTH

Phosphate (increases excretion)

What do the laboratory values tell us about processes occurring in the bone?

• 25-(OH)-D3 = 5 ng/ml (nl = 30-75)

• Serum calcium = 7.5 mg/dl (nl = 8.5-10.5)

• Serum phosphorus = 1.5 mg/dL (nl = 2.5-4.5)

Phosphorus decreased due to low Ca -> increases PTH to increase Ca retention-> increases PTH which decreases _

PTH

How will the loss of 25-(OH)-D3 impact bone homeostasis and change other laboratory values such as PTH?

• Hypocalcemic, missing vit d: Net bone resorption which we can see through the increased THIS trying to maintain Ca levels but it isn’t enough without the vit d (leading to the osteopenia and the fractures)

Classifications for Vit D Deficiencies

0-10 = Severe

10-12 = Moderate

20-30 = Mild

>30 = Normal

A. osteomalacia (lack of Ca and lack of Vitamin D = lack of bone mineralization)

Which of the following bone disorders does Ralph have?

a. Osteomalacia

b. Osteonecrosis

c. Osteoporosis

d. Osteopetrosis

e. Paget’s disease

Celiac Disease! Fat and fat-vitamins are not absorbed! Vit D deficiency!

Which GI conditions may result in similar findings to having decreased vitamin d?

• Vitamin D is a fat-soluble vitamin. It’s absorbed the same way lipids are (emulsify, enzymes (lipase), micelles, diffusion, apolipoproteins added, chylomicrons, lacteals, circulation).

D. A decrease in 1α-hydroxylase activity (without this, no vit d in active form, so no Ca)

Calcium levels in this case of renal failure may be decreased due to which cause?

a. A decrease in serum PTH levels

b. An increase in urinary phosphate levels

c. Inability to produce 25-hydroxyvitamin D

d. A decrease in 1α-hydroxylase activity

B. Renal phosphate clearance decreases in renal failure (because of a decrease in urinary output, phosphate isn’t being cleared, no phosphate cleared means no Ca can be resorbed. Ca and phosphate complex and that leads to renal distruction!)

Bone loss associated with renal failure occurs in part because:

a. The failing kidney is no longer capable of PTH production

b. Renal phosphate clearance decreases in renal failure

c. The failing kidney increases activation of vitamin D

d. Renal calcium clearance increases in renal failure

Chronic Renal Failure leads to:

• Decreased vitamin d activation

• decreased GI calcium absorption

Also

• Increased phosphate retention

• increased serum phosphate

• decreased serum Ca

• Increased PTH secretion

• Increased bone resorption

B. Autoimmune destruction of her parathyroid glands. (no functioning glands means no PTH)

Someone with hypothyroidism has low serum calcium levels due to:

a. autoimmune destruction of her pancreas.

b. autoimmune destruction of her parathyroid glands.

c. inadequate calcium in her diet.

d. inappropriately low 25(OH) vitamin D level.

e. PTH resistance.

Bone resorption

Increased alkaline phosphatase & urine n-telopeptides means increased_

Osteoporosis

Promoted by:

• age, loss of estrogen (post-menopausal), smoking, not enough Ca intake

Prevented by:

• exercise, 25-hydroxy vit d is normal, weight gain (the weight gain and resistance is putting stress on bone, promoting more bone formation (hence the increased n-telopeptides))

Glucocorticoids

Have detrimental effect on bone mass.

We need the perfect amount of cortisol.

THESE can:

• Decrease growth factors

• Decrease osteoblast development

• Increase osteoblast death

• Decreases osteoprotegrin (normally shuts down bone resorption)

Overall, they decrease bone formation, increase bone resorption, result = osteoporosis!

Pharmacologic Osteoporosis Treatments

Two categories:

• Antiresorptive: inhibit bone resorption (i.e. bisphosphonates)

• Anabolic: increase bone formation (i.e. subcutaneous PTH injections)

Don’t use these 2 together! Increased formation increases resoprtion and vice-vera (balancing act).

Bone Resorption

Why is it important to check vitamin D levels prior to starting an osteoporosis treatment?

• With the treatments, we’re inhibiting _ (which limits serum Ca leves). Low Vit D would do the same thing. Not good.

Main targets for treating osteoporosis:

• Maintain bone mass

• Inhibit bone loss

• RANKL

• Sclerostin (antibodies against this inhibit bone resorption) (net formation effect)

Alkaline Phosphatase

Increased _ means unregulated osteoblast activity!

Osteoblasts

Origin of Paget’s?

• THESE!

• Releasing too much alkaline phosphatase

Bone formation

What causes progressive pain in the shins, knees, and left arm in Paget’s Disease?

• Due to increased bone density! Osteoblasts aren’t regulated. Too much_

Bone weakness

Hypervasculature

What causes tenderness above the left elbow, enlarged, bowed shins and warmth over both shins in Paget’s disease?

• Increased _ in this case because of haphazard bone formation!

• Hence bowing and pain!

• Warmth due to _ (more blood)

D. Inhibition of bone resorption

Considering currently available therapeutics, which option should be utilized in the treatment plan for Paget’s?

a. Calcium supplementation

b. Increase in urine output

c. Inhibition of bone formation

d. Inhibition of bone resorption

e. Inhibition of PTH secretion

PTHrP

Ca

Why does this pt have hypercalcemia?

• _ related to malignancy

Why are the PTH levels 2?

• PTH is low because _ is high!