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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SIADH adrenal insufficiency hyponatraemia SST cortisol 345 inadequate response management 2025
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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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