Electrolytes and Volume

1 Sodium

Sodium loading expands plasma volume and probes hypovolemia as an orthostatic mechanism. It carries a specific ME/CFS caution: intracellular sodium overload has been documented by sodium-MRI, so sodium loading may paradoxically worsen symptoms by increasing ATPase demand to pump the extra sodium out of cells.

1.1 If sodium works

Finding: Hypovolemic POTS

Improvement means low plasma volume was limiting venous return; sodium-driven volume expansion restored preload.

Certainty: Medium for volume expansion in hypovolemic POTS.
Does NOT tell us: why plasma volume was low.
Action: Supports volume expansion as a target; combine with fludrocortisone assessment.
Level of action: Partial root cause if hypovolemic.

Finding: Intracellular Na overload (if sodium WORSENS)

If symptoms worsen on sodium, this is diagnostic for the intracellular sodium phenotype documented by sodium-MRI: adding sodium raises the ATPase demand needed to extrude it, worsening the cellular energy deficit.

Certainty: Low to Medium — anchored in sodium-MRI findings; the worsening response is a direct signal.
Does NOT tell us: the cause of the intracellular sodium accumulation.
Action: Stop sodium loading; redirect away from volume strategies toward cellular energetics.
Level of action: Diagnostic redirection (potentially harmful if continued).

1.2 What a positive response does NOT reveal

Why plasma volume is low.
Whether a subclinical intracellular sodium burden coexists.

1.3 If sodium does NOT work

Hypovolemia may not be the mechanism.
Venous pooling or ganglionic impairment may predominate.
An intracellular sodium burden may offset the benefit of volume expansion.

1.4 Key caveat

Because intracellular sodium overload is documented by sodium-MRI in ME/CFS, sodium loading must be started cautiously. If symptoms worsen on sodium, this is itself diagnostic for the intracellular Na phenotype, and sodium should be stopped.

1.5 How sodium combines with other medications

Sodium + fludrocortisone both work → volume expansion confirmed from two angles.
Sodium WORSENS + pyridostigmine works → intracellular Na overload phenotype with ganglionic autonomic failure.
Sodium helps + ivabradine works → a mixed hypovolemic plus hyperadrenergic subtype.

1.6 Compendium

Sodium does not yet have a dedicated entry in the Chapter Mechanistic Cascade Tracing: From Hypothesis to Clinical Probe pharmacodiagnostic compendium. The volume expansion and POTS pathways are detailed at Mechanistic Cascade Tracing: From Hypothesis to Clinical Probe (sec-12, Fludrocortisone entry).

2 Magnesium

Magnesium is a cofactor for more than 300 enzymes, including those governing ATP utilization and the sodium-calcium exchanger (NCX). Magnesium L-threonate crosses the blood-brain barrier. Supplementation probes whether systemic or CNS magnesium deficiency is contributing.

2.1 If magnesium works

Finding: Systemic or CNS magnesium deficiency was contributing

Improvement means a magnesium shortfall was limiting enzyme function or neuronal signaling; repletion helped.

Certainty: Medium for deficiency states — repletion effects are well characterized.
Does NOT tell us: whether the deficiency is dietary, from ongoing loss, or from impaired intracellular trafficking.
Action: Supports checking RBC magnesium and repleting; L-threonate form if CNS effects are sought.
Level of action: Partial root cause.

2.2 What a positive response does NOT reveal

Why magnesium was low — intake, loss, or trafficking.
Whether the deficit is systemic or specifically central.

2.3 If magnesium does NOT work

Magnesium deficiency may not be present — note that serum magnesium is insensitive; RBC magnesium is more informative.
Intracellular trafficking may be impaired, so oral repletion does not reach the deficient compartment.
The deficiency may be secondary to an ongoing loss that outpaces supplementation.

2.4 Key caveat

Serum magnesium is unreliable for total body magnesium — RBC magnesium is more sensitive. Oral magnesium can cause diarrhea; glycinate or L-threonate forms have better tolerability.

2.5 How magnesium combines with other medications

Magnesium L-threonate + creatine both work → converging support for CNS energy metabolism.
Magnesium + mitochondrial supplements both work → multiple energy-pathway cofactor deficits.
Magnesium works + fludrocortisone/sodium does not → a magnesium-specific deficit, not general hypovolemia.

2.6 Compendium

The full pharmacodiagnostic entry — including mechanism-exclusion logic, dose-specific side-effect diagnostic patterns, combination diagnostics, and worsening risk profiles — is at Mechanistic Cascade Tracing: From Hypothesis to Clinical Probe (sec-12, Magnesium entry).