Disorders of Calcium, Phosphorus and Magnesium

Vocabulary flashcards covering the metabolism, biological roles, regulatory hormones, diagnostic formulas, and clinical disorders of calcium, phosphate, and magnesium.

Total Body Calcium

The fifth most common element in the body and the most prevalent cation, totaling approximately $1\,\text{kg}$ ($25\,\text{mol}$), with $98\%$ localized in the skeleton and an extra-osseous fraction of $1\%$.

Free Ionized Calcium

The biologically and physiologically active form of calcium circulating in plasma, representing approximately $50\%$ of total plasma calcium ($2.15\text{--}2.55\,\text{mmol/L}$).

Albumin-Adjusted Calcium

A calculated estimate of the active fraction of total plasma calcium that corrects for deviations in plasma albumin concentration using the formula: $\text{Plasma albumin-adjusted calcium (mmol/L)} = \text{measured calcium} + (40 - \text{plasma [albumin]}\,(\text{g/L})) \times 0.02$.

Parathyroid Hormone (PTH)

A single-chain polypeptide containing $84$ amino acids (with biological activity localized to its $34$ N-terminal residues) secreted in response to low ionized calcium to stimulate osteoclastic bone resorption, increase renal calcium reabsorption, and decrease renal phosphate reabsorption.

Parathyroid Hormone-related Protein (PTHrP)

A peptide hormone that shares an amino acid sequence homology with PTH at its biologically active end, thereby activating the same receptors, and thought to play a role in fetal calcium metabolism.

Calcitonin

A hormone produced by the C-cells of the thyroid gland that decreases osteoclastic activity, slows calcium release from bone, exerts effects opposite to PTH on plasma calcium, and is used to treat hypercalcemia and Paget's disease of bone.

$$1\alpha\text{-Hydroxylase}$$

The proximal renal tubular enzyme that catalyzes the second hydroxylation step converting $25\text{-hydroxycholecalciferol}$ into the active hormone $1,25\text{-dihydroxycholecalciferol}$, stimulated by low phosphate and high PTH levels.

Calcitriol ($1,25\text{-dihydroxycholecalciferol}$)

The active metabolite of vitamin D that enhances intestinal calcium and phosphate reabsorption, increases bone mineralization and osteoclast activity, and inhibits PTH secretion.

Hypercalcaemia

A condition of elevated plasma calcium caused by malignancy, parathyroid abnormalities, high bone turnover, or drugs, associated with ECG changes such as QT interval shortening and carries a risk of sudden cardiac arrest when exceeding $3.5\,\text{mmol/L}$.

Hypocalcaemia

A state characterized by subnormal plasma calcium concentrations resulting in increased neuromuscular excitability (tetany, carpopedal spasm, seizures), cataracts in prolonged cases, and prolonged QT intervals on ECG.

Fractional Phosphate Excretion ($\text{FEPi}\%$)

A laboratory calculation expressed as $\text{FEPi}\% = \frac{\text{urinary phosphate} \times \text{plasma creatinine}}{\text{plasma phosphate} \times \text{urinary creatinine}} \times 100$, where a value exceeding $10\%$ indicates renal phosphate loss.

Hyperphosphataemia

A disorder defined by plasma phosphate levels exceeding $1.45\,\text{mmol/L}$, resulting from tissue breakdown (such as tumour lysis syndrome), renal failure, or hypoparathyroidism, and predominantly producing clinical manifestations secondary to hypocalcaemia.

Hypophosphataemia

A condition characterized by serum phosphate concentration $< 0.87\,\text{mmol/L}$ due to cellular redistribution, reduced intake, or renal wasting, typically warranting treatment when levels fall below $0.30\,\text{mmol/L}$.

Total Body Magnesium

The total reserve of magnesium in the body amounting to approximately $1\,\text{mol}$ ($\approx 25\,\text{g}$), located predominantly within bone and muscle, functioning as an essential cofactor for enzymes and an antagonist to calcium.

Hypermagnesaemia

An elevation in serum magnesium concentration above $1.2\,\text{mmol/L}$, presenting clinically with prolonged PR intervals, widened QRS complexes, increased T-wave amplitude, hypotension, and respiratory depression.

Hypomagnesaemia

A deficiency state marked by serum magnesium levels below $0.80\,\text{mmol/L}$ (severe when $< 0.5\,\text{mmol/L}$), manifesting as anorexia, weakness, arrhythmias, and personality changes, which can lead to refractory hypokalaemia and hypocalcaemia.

Calcium Gluconate

An intravenous therapeutic agent administered as $10\,\text{mL}$ of a $10\%$ solution over $5\text{--}10\,\text{minutes}$ for life-threatening manifestations of hypocalcaemia (tetany, seizures) and severe hypermagnesaemia.