What was the diagnosis in this case of an infant with excessive thirst?

Case of the month: GP and paediatrics specialist Dr David Capehorn asked readers what the next steps and likely diagnosis could be in this tricky case of an infant presenting with excessive thirst and urination, but otherwise normal behaviour and normal glucose on a urine dip. Did you get the diagnosis? Answer revealed below!

A 2½-year-old boy is brought into your surgery because, as his mother explains, ‘he’s drinking loads of water’. She says he is constantly thirsty, asking for drinks repeatedly throughout the day and night. His nappies are ‘always soaked and leak’, yet his behaviour seems otherwise entirely normal. There has been no weight loss, no fever and no vomiting, and his appetite is unchanged.

These symptoms have crept in gradually over several months. His mother wonders whether it has simply become a habit, but adds that he becomes very irritable if she tries to limit drinks; she is also worried he could have diabetes and has brought a specimen of urine for you to test. She doesn’t know how much he is drinking in total, as he also goes to nursery. There is no history of constipation, no previous UTIs, and he had an uncomplicated birth after an uneventful pregnancy. There is no relevant family history.

In the consulting room, he appears bright, interactive and well. Observations are normal. There are no findings on general or systemic examination, and his weight plots on the 25th centile, although the child’s red book isn’t available to check any trend.

You carry out a urine dipstick test which indicates normal glucose levels and no ketones.

Given the child is otherwise well and behaving normally, you reassure the mother that he seems well and that no further tests are needed for the time being, advising her to come back if anything changes.

Within two weeks, the mother brings the child back in, saying she is worried about his ongoing thirst and nappy soaking. She is not sure, but feels it might be getting even worse. Either way, she says, this is ‘really not normal’ and she is convinced there is something wrong.

Given the mother’s renewed concerns, and before specialist referral, what are the most informative next investigations to perform in primary care? And what is the likely diagnosis?

Answer: You perform a capillary glucose test which is normal, and a urine dipstick test which you scrutinise more carefully and which shows very low specific gravity (SG <1.005), no glucose, no ketones, and no evidence of infection (no white cells or nitrites).

Repeat urine samples on different days show the same very dilute urine. The persistent polydipsia, polyuria and markedly low specific gravity raise significant concern about a urine-concentrating defect.

Following national paediatric guidance, you arrange blood tests including U&Es (with particular attention to sodium), creatinine and urea, calcium, glucose, and paired urine and serum osmolality. These results show high-normal serum sodium, high serum osmolality, and very low urine osmolality.

The combination of concentrated blood alongside very dilute urine suggests diabetes insipidus (DI), and you refer the child urgently (the same day) to paediatrics. There, he undergoes a supervised desmopressin challenge, detailed serum and urine osmolality testing, and genetic analysis.

Results show a minimal response to desmopressin, and genetic testing confirms X-linked congenital nephrogenic diabetes insipidus (NDI).

The specialist team initiates a low-solute diet (low salt and controlled protein), a thiazide diuretic to reduce urine volume, ongoing unrestricted access to fluids, monitoring of growth and renal function, and education for the family on dehydration risks and managing intercurrent illness.

Over the following weeks, nighttime waking improves, nappies stop overflowing, the child is calmer and less irritable, and growth improves.

Learning points for GPs

This case illustrates how easily pathological polyuria and polydipsia in young children can be mistaken for behavioural drinking, particularly when the child appears otherwise well. In this toddler, the key clues only became apparent when symptoms persisted and were reviewed more critically at the second consultation.

GPs should consider diabetes insipidus (DI), including nephrogenic DI (NDI), when a child presents with persistent thirst and excessive urine output, particularly if this includes waking overnight to drink. In this case, repeated reports of soaked nappies and irritability when fluids were limited were important historical features.

Simple urine testing in primary care can be highly informative. A persistently very low urine specific gravity (<1.005), in the context of normal glucose and no evidence of infection, should prompt concern about a urine-concentrating defect. Early onset of symptoms in infancy or toddlerhood, as seen here, strongly suggests nephrogenic DI, and a family history of affected males may be present.

Although DI is rare, it is potentially dangerous if missed. Children may maintain normal appearances for some time, but are at risk of hypernatraemia, dehydration, renal injury and impaired growth. For this reason, fluids should never be restricted in primary care.

Red flags requiring urgent same-day paediatric referral

GPs should arrange urgent referral if any of the following are present:

Hypernatraemia (>150 mmol/L)
Polyuria and polydipsia with vomiting, fever or reduced oral intake
Faltering growth or hydration concerns in infants
Lethargy, tachycardia or clinical signs of dehydration

Appropriate investigations in primary care

Initial assessment should focus on safe, accessible tests:

Urine dipstick, with particular attention to specific gravity.
Capillary blood glucose.
Blood tests including U&Es, calcium, creatinine and glucose.
Paired serum and urine osmolality, where available.
Accurate growth measurements.

What not to do in primary care

Do not restrict fluids.
Do not perform a water deprivation test outside hospital.
Do not trial desmopressin in primary care.

Key points

Excessive drinking in toddlers should not automatically be attributed to behavioural causes.
Persistently dilute urine in a polyuric child with normal glucose is a critical diagnostic clue.
Early recognition and timely referral, as demonstrated in this case, can significantly improve symptoms and prevent serious complications.

Dr David Capehorn is a GPwSI in Paediatrics and Honorary Associate Specialist, Bristol Children’s Hospital.