Endocrine input Sheppard ST4 Remote review of notes Hyponatraemia Na 1

Endocrine input Sheppard ST4 Remote review of notes Hyponatraemia Na 115 (dropped from 128 pre-op) Left NOF- left hemiarthroplasty on 15/06/26 Serum osm 251 - pending urine osm and urine Na Reported to be clinically euvolaemic - given 1L IVI yesterday, pending repeat bloods Cortisol 161 Imp: Acute hyponatraemia - suspected SIADH or adrenal insufficiency Suggest: - if symptoms of hyponatraemia including seizures, drowsiness, vomiting then would need hypertonic saline - chase results of SST, if inadequate response to SST (cortisol <450 ) needs to start hydrocortisone 10mg, 5mg, 5mg - if SST normal and urine Na >30 likely SIADH and please fluid restrict to 1L per day in the first instance - twice daily U&Es while Na <120 - if urine Na <30 likely to be either hypervolaemic or hypovolaemic and this needs to be reviewed with today's bloods to see if there has been a response to giving IV fluids . Severe Hyponatraemia, Endocrine input 15/07 noted Urine Na 136, U osmolality 506, P osmolality 251 Cortisol 274->SST->345 Commenced on hydrocortisone 10mg-5 mg- 5mg as per endocrine advise

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Clinical Review - Endocrine Input: Hyponatraemia (Ward ST4 / Sheppard)

Date of review: 16 July 2026 | Remote review of notes

Summary of Current Situation

Parameter	Value	Interpretation
Na (pre-op)	128 mmol/L	Baseline hyponatraemia
Na (current)	115 mmol/L	Severe, acute-on-chronic drop of 13 mmol/L
Serum osmolality	251 mOsm/kg	Hypotonic (confirms hypotonic hyponatraemia)
Urine Na	136 mmol/L	High - not sodium-depleted state
Urine osmolality	506 mOsm/kg	Concentrated urine - ADH-mediated water retention
Cortisol (random)	274 nmol/L	Suboptimal for a physiologically stressed post-op patient
Post-SST cortisol	345 nmol/L	Inadequate response (threshold <450 nmol/L)

Interpretation of SST Result

The SST result (274 → 345 nmol/L) is clearly inadequate:

A normal/adequate SST response = peak cortisol ≥450 nmol/L at 30 or 60 minutes post-ACTH
The increment is only +71 nmol/L and the peak is well below 450 nmol/L
A baseline cortisol of 274 nmol/L in a post-operative, physiologically stressed patient (NOF fracture + hemiarthroplasty) is also inappropriately low - one would expect cortisol >500 nmol/L in significant physiological stress
This confirms relative/secondary adrenal insufficiency as the primary driver of the hyponatraemia

The urine profile (U-Na 136, U-Osm 506) mimics SIADH biochemically - this is expected in adrenal insufficiency, where glucocorticoid deficiency leads to non-osmotic AVP release and impaired renal free-water excretion. ACTH/cortisol deficiency elevates AVP independent of osmolality, and glucocorticoid replacement directly suppresses AVP and corrects this.

The clinical team has done the right thing commencing hydrocortisone 10mg-5mg-5mg as per the original endocrine advice.

Key Points for Ongoing Management

1. Hydrocortisone - confirm correct regimen

10 mg morning / 5 mg early afternoon / 5 mg early evening = 20 mg total daily
This is an appropriate physiological replacement dose (standard recommendation is 15-25 mg/day in divided doses; Bailey & Love, Harrison's 22e)
Post-operatively, consider whether stress dosing is still needed: for a patient day 1 post-NOF hemiarthroplasty, many endocrinologists would use 50-100 mg/day hydrocortisone (stress dose) for 24-48 hours post-op before stepping down to 20 mg maintenance. If the patient is haemodynamically stable and not systemically unwell, 20 mg/day is reasonable but monitor closely
No fludrocortisone needed at this dose - hydrocortisone at 20 mg has sufficient mineralocorticoid activity (fludrocortisone only added when daily hydrocortisone dose is below ~50 mg and primary adrenal insufficiency is confirmed)

2. Na correction - safety limits

Na 115 mmol/L is severe hyponatraemia
Even though this appears to be acute (dropped from 128 over a short peri-operative period), caution is needed:
- Do not exceed 10-12 mmol/L correction in 24 hours
- Do not exceed 18 mmol/L in 48 hours
- Risk: osmotic demyelination syndrome (ODS / central pontine myelinolysis) with overcorrection
Hydrocortisone alone may correct Na by driving free-water excretion - monitor closely to ensure auto-correction does not overshoot
The 1L IVI given yesterday needs to be reconsidered - IV fluids are appropriate if hypovolaemic but may accelerate overcorrection now that hydrocortisone is started

3. Monitoring plan

Twice-daily U&Es until Na >120 mmol/L (as originally advised) - this is correct
Once Na is rising with hydrocortisone, be vigilant for rapid overcorrection, especially as the adrenal insufficiency is treated and ADH suppression occurs abruptly
Target: Na rise of no more than 8-10 mmol/L per 24 hours
If Na corrects too rapidly: consider DDAVP 2 mcg IV/SC + 5% dextrose infusion to intentionally re-lower Na ("rescue strategy")

4. Symptoms to watch for

If the patient develops seizures, reduced consciousness, vomiting - hypertonic saline (1.8% or 3% NaCl) is indicated regardless of aetiology (150 mL 3% NaCl over 20 min, repeat as needed, target 5 mmol/L acute symptomatic relief)
Conversely, watch for symptoms of rapid overcorrection: new confusion, dysarthria, dysphagia emerging days 2-3 post-treatment

5. Fluid restriction

With adrenal insufficiency confirmed, fluid restriction is less relevant now - this was the fallback for pure SIADH
Do not restrict fluids if the patient appears hypovolaemic or has any haemodynamic concern (risk of dehydration in post-operative orthopaedic patient)

6. Further investigations to consider

If not already done: morning cortisol 48-72 hours after starting hydrocortisone is not useful (suppressed by treatment) - formal reassessment of the HPA axis should be deferred to outpatient follow-up 6-8 weeks post-discharge
Consider whether this is primary or secondary adrenal insufficiency - ACTH level (taken before starting hydrocortisone if possible), looking for other pituitary hormone deficiencies if secondary
Thyroid function (hypothyroidism also causes euvolaemic hyponatraemia and should be excluded if not already done)

Working Diagnosis

Relative adrenal insufficiency (likely secondary/central) causing euvolaemic hypotonic hyponatraemia - the elevated urine Na and concentrated urine are consistent with this. The SST response of 345 nmol/L confirms inadequate cortisol reserve.

SIADH may coexist (e.g. post-operative pain, nausea, analgesia as ADH secretagogues), but the cortisol findings provide a sufficient and treatable explanation as the primary driver.

Red Flags to Escalate

Na failing to rise within 12-24 hours of hydrocortisone - may need dose increase or re-evaluation
Haemodynamic instability - may need stress-dose hydrocortisone (100 mg IV bolus)
Na rising >10 mmol/L in first 24 hours - act immediately to slow correction
New neurological symptoms in either direction

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