My doctor said I "almost" have an underactive thyroid, which puzzled me. Then she said I might benefit from treatment, which puzzled me more. How can you "almost" have a disease, and how can treatment be useful if you don't have the disease?
I can see how that might have been confusing. In fairness, it's a confusing situation, even for doctors. Let's take this in two parts. First, what's an "underactive thyroid"? The thyroid gland in our neck makes a hormone, called thyroid hormone, that travels throughout the body and affects every cell. If it is making the right amount of the hormone — if the blood levels are normal — then there's no problem. If it's making too much (an overactive thyroid), or if it's making too little (an underactive thyroid), those are problems that require treatment.
We used to have one blood test to tell if a person had a normal, overactive, or underactive thyroid. However, the test had its limitations, so a second blood test was developed. The newer test measures whether the brain thinks thyroid hormone levels are normal. Usually, the results of the two tests point in the same direction: both say "normal," "overactive," or "underactive."
But sometimes the two tests give different answers: the newer test says "underactive," but the older test says "normal." Doctors call this "subclinical hypothyroidism," and I'm pretty sure that's what your doctor thinks you have. It's quite common: nearly 20% of people ages 65 or older, particularly women, have it. And some of these people also have symptoms that could be caused by an underactive thyroid gland, such as fatigue, constipation, feeling cold, slowed movements and speech, and unexplained weight gain.
Second, does treatment help in people with subclinical hypothyroidism? In 2017, the results of a large randomized trial involving over 700 people with this condition (ages 65 and older) were published in The New England Journal of Medicine. Some received thyroid hormone and others received placebo pills, and neither they nor their doctors knew which they'd been given. Over the next year, the people who received thyroid hormone had symptoms and quality of life similar to those who received the placebo. In other words, the treatment produced no apparent benefit. It also caused no apparent adverse effects, at least for the year of the study.
This study shows that the average person with subclinical hypothyroidism does not benefit from thyroid hormone pills. Perhaps future studies will focus on patients with more severe symptoms, since it is possible that treatment might benefit them. For now, it's just one more important thing that doctors don't know, and need to know — through research.
Harvard Health Letter
