The couple sat in silence as they drove home from Blank Children’s Hospital in Des Moines. Her 5-month-old appeared to be dwarfed by the baby carrier strapped to the back seat. He was tiny. He hadn’t grown since he was 2 months old. He weighed just 10 pounds – barely three pounds more than when he was born.

The baby was breastfed from birth, but his mother immediately noticed that he struggled with it more than his three older brothers. She tried putting her breast milk in a bottle to see if that would be easier for her baby to handle. When that didn’t help, she tried adding baby food. He often spat; sometimes it seemed as if more was coming out than was going in. His pediatrician prescribed him an acid-reducing medication. It didn’t seem to do much either.

Despite his size, he looked healthy. He was active. He was able to achieve all of his milestones. He could hold his head up. He could turn around. His fontanelle, the soft spot on his head, was flat—as it should be. His pediatrician advised patience, but when the boy still hadn’t gained any weight at his 4-month visit, she sent blood samples to the lab.

In the late afternoon, the parents received a call with the results. The baby had worrisome abnormalities in his blood chemistry. The salt level in his blood was very high, so high that he could trigger a seizure. In fact, it was so high that he could die if not addressed. Parents rushed the little boy to Blank Children’s Hospital.

Samples taken in the hospital’s emergency room quickly confirmed the anomaly. The child’s sodium level was 159, more than 10 points above normal. The high number not only told his doctors that he had too much sodium, but also that he didn’t have enough water in his body, that he was very dehydrated.

Normally, when there is too much sodium in the body, the brain triggers the urge to drink in order to absorb more water. The brain also tells the kidneys to retain as much water as possible.

The brain communicates all of this with a hormone called vasopressin. Problems with vasopressin can cause a condition first described in the 1700s as diabetes insipidus (DI) — a disease that produces copious and watery (rotten) urine.

The combination of the child’s high sodium levels and watery, dilute urine immediately led doctors to suspect he had DI. His high sodium levels should have caused his brain to send a vasopressin message to his kidneys to hold on to as much water as possible. And yet his urine consisted almost entirely of water. Why? Wasn’t the pituitary gland in his brain able to make the hormone? Or was there a problem on the news-receiving side in his kidneys?

No matter where the problem started, there were medications that could help. Doctors give the baby two drugs that are normally used to control high blood pressure and that cause the kidneys to excrete sodium. Almost immediately, the baby’s sodium began to drop. This indicated that the baby had DI. If so, was the problem in the brain, where the hormone is made, or in the kidneys? How the problem was handled depended on where it originated.

An MRI was done to look for signs of a problem in the pituitary gland. It looked normal. The problem, his doctors thought, was probably in his kidneys. They sent samples to look for a genetic reason for his abnormality, but those results would not be available for weeks.

In the meantime, they continued to give the baby the medicines that helped him get rid of the salt. And slowly the levels dropped. After a few days on these drugs, the baby’s chemistry was perfectly normal. His parents were told he should start gaining weight now. But he did not do it. By the day the doctors decided the baby was well enough to go home, it still hadn’t gained an ounce.

Parents were instructed to feed the child every three hours 24/7 to help him get the maximum number of calories. They should contact their pediatrician and see a genetics specialist. Then they were sent home. They had a strong feeling that their baby was not ready to leave the hospital. He was brought in with a diagnosis of failure to thrive and he’s still not doing well. He was in the zero percentile on the growth chart. Zero. They brought this argument to the boy’s doctors. He will gain weight now that his chemistry is normal they were told. Just give him time.

The child’s parents felt that he did not have time, that his life was still in danger. So early the next morning, parents and baby were back in the car. They had talked their way through to an appointment with the genetics specialists at the University of Iowa Stead Family Children’s Hospital in Iowa City, two hours away. When they got there, the parents shared their concerns. Was the baby’s inability to gain weight because of his DI? Or was there something else going on?

The child had been tested for cystic fibrosis at Blank Hospital. The test was inconclusive. Dozens of other medical conditions can affect a baby’s growth. Parents and baby were sent to the lab to have blood drawn to check for other genetic abnormalities and to the cardiology department to make sure his heart was normal.

The geneticist also wanted the baby to be evaluated by a pediatric gastroenterologist. It was clear he was having trouble feeding and seemed to spit out much of what he was able to eat. The geneticist turned to Dr. Eyad Hanna, who saw the child later that day. It was only minutes before the gastroenterologist decided the child was too small to send home. Like the child’s parents, he feared that if the baby couldn’t gain weight in the hospital, he might not be able to make it at home either. The baby was taken into Hanna’s care and fed around the clock to try to help him get back on the growth curve. Hanna also turned to a pediatric kidney specialist, Dr. Pat Brophy, who recommended adding plain water to make up for the water the boy lost with his urine. Doctors usually advise mothers not to give their babies water because breast milk contains enough water. But this clearly wasn’t a normal baby. And because of the reflux and difficulty breastfeeding the baby, Brophy also recommended placing a tube in the baby’s stomach — a gastrostomy, or G-tube — to ensure it could get enough calories, medication, and much-needed extra water.

The baby continued to spit up copious amounts of the milk and water he was given. Usually, this type of spitting goes away as an infant’s esophagus lengthens and stomach enlarges. But this baby would not grow at all without more food. Hanna recommended adding baby food and dry food to the milk. He had her enlarge the hole in the bottle’s nipple so the thickened liquid could flow through easily.

And then they waited. Test results trickled in. He didn’t have cystic fibrosis. His heart was perfectly normal. But even as the negative results began to roll in, the baby’s parents could see that he was doing better just because he was getting the calories and most importantly, the water he needed. Every night he got the equivalent of an 8-ounce glass of water through his G-tube. Every day he was fed every three hours to get a total of 1,300 calories. And slowly he started gaining weight – 30-40 grams per day. He stayed in the hospital for almost two weeks, and by the time he and his parents were able to go home he had gained over a pound. He needed a few more months to get back on the growth curve. Only then did they get the results of the genetic test, which confirmed what they already knew: the baby had DI

This baby is now 7 years old. He’s learning to live with his DI. He continues to take the medications that help him get rid of his sodium. He often has to go to the toilet. And he has to drink lots and lots of water to replace whatever he loses in his urine. He’s not as big as his brothers – not yet and maybe never. But he’s still growing and thriving, and that’s more than enough for his parents.

Lisa Sanders, MD, is a contributing writer for the magazine. Her latest book is Diagnosis: Solving the Most Baffling Medical Mysteries. If you have a solved case you want to share, write to her at Lisa.Sandersmdnyt@gmail.com.