Six Litres of Urine a Day: Is It Sugar Diabetes?
Marcus, 14, is brought to a health centre. Over the past month he has been drinking constantly and passing up to 6 litres of urine a day, waking four or five times a night; he has lost 3 kg although his appetite is good. His fasting blood glucose is 4.6 mmol/dm³ (reference 3.9–5.5), and a urine dipstick is negative for glucose and for ketones. His urine specific gravity is 1.002 (usual 1.005–1.030). A water-deprivation test is started: after six hours with no fluid he has lost 2.5% of his body mass and his urine specific gravity has risen only to 1.003. His blood sodium is at the upper end of normal. His aunt is convinced he has 'sugar diabetes' and wants the family to remove all sweet foods; a neighbour says he is simply drinking too much out of habit and should be made to stop. The health centre has no specialist and no desmopressin in stock.
1. Explain what a normal response to water deprivation should be, using ADH and negative feedback, and interpret Marcus's result.
Read the worked answer
Normally, a fall in blood volume and a rise in the osmotic pressure of the blood are detected by osmoreceptors in the hypothalamus. The posterior pituitary releases more ADH, which makes the walls of the distal tubule and collecting duct more permeable to water, so more water is reabsorbed into the blood and a small volume of concentrated urine is produced - specific gravity rises above about 1.020. Thirst is stimulated at the same time. As water is replaced, ADH secretion falls: that is negative feedback.
Marcus lost 2.5% of his body mass over six hours and his urine specific gravity stayed at 1.003, barely different from 1.002. His kidneys did not concentrate his urine even while he was becoming dehydrated, which is not a normal response. This points either to ADH not being produced or released in adequate amounts (cranial diabetes insipidus) or to the kidney not responding to it (nephrogenic diabetes insipidus). The large volume of dilute urine and the night waking fit that interpretation. Further tests - plasma osmolality, an ADH measurement and a supervised trial of desmopressin - would distinguish the two.
2. Using the data, distinguish between diabetes mellitus and Marcus's condition. Evaluate the aunt's and the neighbour's advice.
Read the worked answer
In diabetes mellitus the problem is insulin, so blood glucose is high (fasting above about 7 mmol/dm³) and glucose spills into the urine, often with ketones; the glucose in the filtrate holds water in the tubule and causes osmotic diuresis. Marcus's fasting glucose is 4.6 mmol/dm³, squarely inside the reference range, and his urine is negative for both glucose and ketones. The aunt's diagnosis is therefore not supported: this is not 'sugar diabetes', and removing sweet foods will not reduce his urine output. It would also be unhelpful for a growing 14-year-old, who needs a normal balanced diet.
The neighbour's advice is worse and could be dangerous. Marcus is not drinking out of habit: he drinks because he is losing several litres of water a day that his kidneys cannot conserve. If he is made to cut down, his blood becomes more concentrated - his sodium is already at the top of the range - and dehydration can progress to confusion, seizures and collapse. He should be allowed free access to water until he has been properly assessed.
3. Explain why Marcus is at risk of dehydration even though his blood sodium is only at the upper end of normal, and outline three things the health centre should do while arranging referral.
Read the worked answer
His sodium at the upper end of normal is an early warning rather than a reassurance. With a fixed loss of up to 6 litres of dilute urine a day, any day on which intake falls - illness, vomiting, a long journey, or being told to stop drinking - tips the balance so that water loss exceeds intake. Plasma volume falls and plasma osmotic pressure rises; because the urine is dilute, he loses relatively more water than sodium each time he passes urine, so the sodium concentration creeps upward. Symptoms can escalate quickly in a 14-year-old, and if he cannot drink he may need intravenous fluid.
While arranging referral the centre should:
- Allow free fluids and record daily intake, urine volume and body mass, with urine specific gravity - this shows whether he is keeping up and gives the specialist useful data.
- Advise against high-salt foods, fizzy drinks and sugary drinks, which add solute without helping, and give the family written safety-net advice: return immediately if he vomits, becomes drowsy, has a headache, or stops passing urine.
- Refer urgently to a paediatrician or endocrinologist and check whether desmopressin can be obtained; if it is supplied, it must be used only under supervision with fluid monitoring, because over-treatment causes water retention and low blood sodium.