The 61-year-old woman lifted the laundry basket, then grimly eyed the steps up from the basement. The climb seemed to get tougher every day. Before she was even halfway up, she could feel her racing heart and hear her ragged, rapid breathing. She gripped the wooden handrail in case she started to feel lightheaded again. Once she made it up to the main floor of her home, she dropped her burden and practically fell into the soft embrace of her living-room sofa.

The woman first noticed that she often felt out of breath a few months earlier. She wondered if it was her weight. She liked the way she looked, but suspected that her usual care provider, a much-trusted nurse practitioner with a medical group at Yale School of Medicine, would be happy if she lost a few pounds. Her daughter worried that her shortness of breath could be a result of the Covid infection she had a couple of years earlier.

She finally called her P.C.P.’s office when her right leg started to hurt. The combination of shortness of breath and leg pain made her nurse practitioner worry that the patient had a clot in her leg that had broken off and lodged in her lungs. She sent her to get an ultrasound of the leg. There was no clot, which was a relief, but she was still out of breath.

Referral to the Long Covid Center

From that point on it seemed as if she had a video consultation just about every week. Each visit focused on the newest of her many varied symptoms. One week she was so dizzy it was hard for her to walk. The next week she shared her concerns about her recent weight loss: 10 pounds in just two weeks. Another time she saw the physician who worked with her nurse. As she faced her across the distance of their linked computers, the doctor suggested that the lightheadedness might come from not drinking enough water. Her shortness of breath and rapid heart rate were probably because of a lack of exercise. At the next video visit, her nurse practitioner said she might have POTS — short for postural orthostatic tachycardia syndrome. In this uncommon disorder, patients have a rapid heart rate, or tachycardia, whenever they stand up. It is sometimes seen in patients after they recover from a Covid infection.

With every new concern, there were more tests. She had ultrasounds and chest X-rays; an echocardiogram showed that her heart was pumping normally; and something called a Holter monitor, a wearable device that tracks heartbeats for 24 hours, showed only that her heart sometimes beat rapidly. She had an M.R.I. of her brain, a CT scan of her chest and lots and lots of blood tests. After all this, the patient was frustrated and frightened that no one could explain why she suddenly felt so bad in so many ways. She was referred to a neurologist, a cardiologist and Yale’s Long Covid Multidisciplinary Care Center.

She was seen first at the Long Covid Center, where I am the medical director. That April morning, she told me that she was out of breath all the time. She had to quit her job as a cashier. Her eyes grew shiny when she described her rapid decline. Her heart and lungs had already been evaluated extensively, and the only abnormality was seen in the Holter-monitor test. Like her nurse practitioner, I wondered if she had developed POTS. Certainly, POTS was something I saw regularly in patients with long Covid. A recent study estimated that up to 14 percent of patients developed POTS after recovering from Covid-19.

This disorder can be diagnosed using something called the active-stand test. In this test, the patient’s heart rate and blood pressure are monitored when they are supine and then as they stand in place for 10 minutes. A diagnosis of POTS is made if the blood pressure remains stable while the heart rate increases by at least 30 beats per minute.

When the patient was given the test, she became lightheaded and out of breath after only a few minutes, and the test had to be stopped early. Her heart rate had increased — to 140 from 101. POTS is not well understood but is thought to signify an injury to the autonomic nervous system. Normally the act of standing triggers a rapid increase in blood moving from the legs and lower body up to the heart so that it can be redistributed to the upper body and brain. In POTS, that doesn’t happen, and to get enough blood to the brain, the heart must beat faster — usually a lot faster. In this patient’s case, as in many others I’d seen, that increase in heart rate is inadequate and the patient feels lightheaded. POTS often appears after a viral infection. While there are medications that can help, the first line of treatment is to increase the amount of water stored in the body and to use compression garments to help get blood out of the lower extremities back to the heart.

I wasn’t sure if all of this patient’s symptoms could be attributed to POTS but thought treating POTS was the right first step. I encouraged her to drink more water and prescribed the needed garments. Two weeks later, when she reported that her heart was still racing, I started her on a medication to slow it down.

She returned to the office a couple of weeks after that. Her heart was better, she told me, but she was losing weight: “When I look in the mirror, I barely recognize my face.” Her normally plump, round cheeks looked flat and drawn. She had lost 25 pounds in three months. That stopped me. Weight loss was not a usual symptom of POTS. Had I gotten the diagnosis wrong?

An Essential Step Skipped

Suddenly her symptoms took on an entirely different shape. I had been making assumptions about her case based on the fact that the doctors who referred her to our center thought her symptoms were a result of long Covid. But in truth, none of us really knew if she was suffering from long Covid. There is no definitive test that links present symptoms to a Covid infection experienced weeks, months or even years earlier. The first published reports of symptoms that outlasted the viral infection came in April 2020, just months after the virus arrived in the United States. And since then, a wide range of symptoms has emerged. But the links between the symptoms and the hypothesized cause was temporal. There is only the patient’s experience to suggest a connection. But as with so many of the disorders for which there are no definitive tests, this is a diagnosis that can be made only when other possibilities have been ruled out. In seeing this patient, I skipped that essential step.

This was a middle-aged woman who had a racing heart, who became short of breath with any exertion, who was rapidly losing weight — a classic presentation of thyroid-hormone overload, a condition known as hyperthyroidism. This tiny gland located in the neck is part of a complex system that controls body metabolism. When too much thyroid hormone is released, the body’s engine revs as if someone had stepped on the gas and not let up. She had all the symptoms of hyperthyroidism, and I had simply not seen it. I sent her to the lab down the hall from my office. Within hours it was clear that her system was flooded with these hormones.

I called the patient immediately to explain that, despite the positive active-stand test, she probably did not have POTS and that, instead, her thyroid had gone wild. This is usually a result of an autoimmune disorder known as Graves’ disease, in which antibodies bind to thyroid gland receptors, mistake these normal cells for attacking invaders and trigger a near-continuous release of its hormones. Even before the diagnosis of Graves’ was confirmed, the patient was started on a medication to block hormone production.

I have spent the past 20 years writing and thinking about diagnostic errors. And I understand how this one happened. In medicine, most diagnoses are made through a process of recognition. We see something, recognize it and act on what we see. Most of the time we are right. Most of the time. I’ve asked this patient for a photograph to keep on my desk. A reminder, I hope, that the first diagnosis to come to mind can never be the only one considered.

As for the patient, she feels much better since starting these medications. Her heart rate is down, and the basement stairs are getting easier. She tells me she stopped losing weight, but she won’t feel as if she is back to her old self until her cheekbones go back into hiding.

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