I was at my desk dictating a report when one of our pediatric endocrinologists knocked on my open door.

“Hi, Mark,” she said. “I have a 12-year-old patient whose mother is worried that she may have ADHD [attention deficit hyperactivity disorder]. She says her daughter has been getting more and more inattentive and disorganized and takes a long time to finish her homework. Her lab tests show that she might have resistance to thyroid hormone, which I know can cause ADHD symptoms. I’m not convinced she has that—because it’s pretty rare—but I’m also not sure she has ADHD. Since you know more about ADHD than I do, I’d like to get your opinion.”

“I’ll be happy to take a look. I’ll let you know what I think after I see her.”

The young lady and her mother showed up about a week later, bringing a folder of report cards bearing A’s and B’s.

“How long have you been concerned about her?” I asked her mother.

“For about the past year or so she’s been having difficulty in school,” she replied. “She does her work, but then forgets to turn it in. She tells me it sometimes takes her four hours to do an hour’s worth of homework.”

These are common complaints regarding children with ADHD. ADHD is a chronic condition that causes inattention and difficulty in organizing. Most children with ADHD also show impulsivity and hyperactive behavior, but a sizable number have problems only with attention and focusing. The term ADHD is now used for the hyperactive and non­hyperactive forms of this disorder.

There is no specific laboratory test for ADHD. It is diagnosed by determining if a child has a significant number of inattentive, hyperactive, and impulsive symptoms; if they are excessive for the child’s age; if they have been present since early childhood and in more than one setting; and if they cannot be explained by some other physical, mental, or emotional condition. So I asked the girl’s mother questions designed to detect the symptoms of ADHD.

Her responses showed no hints of impulsivity or hyperactivity. That in itself wasn’t a surprise. Children, especially girls, with the inattentive form of ADHD are often not diagnosed until middle school because of their lack of disruptive behavior. But as I continued questioning, it became clear that this patient did not have ADHD at all. Not only had her symptoms all started within the past year but she did not have enough of the ADHD symptoms to merit the diagnosis. Something else was going on.

Her chart noted that about six months previously her mother had brought up her daughter’s picky appetite and slow growth. Because there was a family history of thyroid problems, the pediatrician had ordered basic blood tests of thyroid function, although she expected they would be normal. In fact, they were not—an important clue.

The thyroid gland, a small butterfly-shaped structure situated low in the front of the neck, controls the growth and metabolism of almost every organ in the body. It produces two related hormones, called T4 (or thyroxine) and T3 (or triiodothyronine). The release of these hormones is triggered by the pituitary gland, which secretes a substance called TSH (thyroid-stimulating hormone) that regulates thyroid production in a feedback loop. In hypothyroidism, the supply of thyroid hormones does not keep up with the body’s demands, and the TSH level is high, reflecting the body’s need to prod for more thyroid production. In hyperthyroidism, when the thyroid gland is producing too much of its hormones, the TSH level drops to zero.

In nearly half the cases of hyperthyroidism in youths, the cause is an autoimmune condition known as Graves’ disease. (The other half are caused by growths on the thyroid.) In Graves’ disease, for some unknown reason the body begins to make antibodies against the thyroid gland, which makes the gland enlarge and produce more thyroid hormones. Typically, someone with this condition will have symptoms of hyperthyroidism, which can include fatigue, heat intolerance, sweating, palpitations (the physical awareness of a rapid heart rate), and weight loss.