If you’re among the 4 to 15 percent of Americans walking around with subclinical hypothyroidism, meaning the beginning signs of an underactive thyroid, you may experience more than the usual mild fatigue, weight gain, dry skin and other slow thyroid symptoms. According to a new review of studies published in The Journal of Clinical Endocrinology & Metabolism, people younger than 75 with subclinical hypothyroidism are at “significant risk” of cognitive impairment and even dementia.
When the researchers, from the University of Pisa, in Italy, examined the results of 13 studies on the effects of subclinical hypothyroidism—defined as elevated serum thyroid-stimulating hormone (TSH) levels and normal thyroxine (T4) levels—they found a 56 percent increased risk of impaired cognitive function and an 81 percent increased risk of dementia. The exact nature of “impaired cognitive function” varied slightly from one study to another depending on the rubric they used—any of several standardized tests such as the Mini Mental State Examination (MMSE), Wechsler Adult Intelligence Scale and the Wechsler Memory Scale-Revised scores.
“Mild, non-specific neuro-cognitive symptoms may be seen in subclinical hypothyroidism,” says James Hennessey, MD, associate professor of medicine at Harvard Medical School and clinical director of the Division of Endocrinology, Diabetes and Metabolism at Beth Israel Deaconess Medical Center in Boston. “Individuals with such symptoms deserve to have their thyroid function tested—and if they are clearly subclinical hypothyroid, levothyroxine therapy should be considered.”
While a number of studies have examined this issue before, this analysis is novel in that it’s the first to consider the age of patients—which dramatically impacted the findings. “It turns out that our TSH levels tend to drift up with age, such that by the time we’re over 80, the upper limit of the TSH has been observed to be over 6 to 7,” explains Dr. Hennessey. “In these studies, people were considered to have subclinical hypothyroidism if their TSH levels were greater than 4 to 5.” In other words, when the previous studies included older adults—whose TSH levels are naturally higher, even in the absence of disease—it skewed the results for the younger adults.
“Diluting the sample with ‘normal’ people made it difficult to demonstrate that subclinical hypothyroidism had an adverse impact on cognition,” Dr. Hennessey continues. “The younger we are, the more likely it is that subclinical hypothyroidism will have an impact on cognition, as shown in this meta-analysis.”
Unfortunately, because the symptoms of subclinical hypothyroidism are either very subtle or not noticeable at all—you could easily brush them off as normal signs of aging, the condition may go undiagnosed for years. When this happens—when your thyroid gland is constantly being stimulated to release more hormones, it could lead to an enlarged thyroid (goiter), and you can become more forgetful, your thought processes may slow, and you may feel depressed, according to the Mayo Clinic. Although advanced hypothyroidism is rare—some symptoms include low blood pressure, decreased breathing and decreased body temperature—it could be life threatening.
Still, there are no official guidelines recommending screening for subclinical hypothyroidism, and that’s unlikely to change anytime soon. In Dr. Hennessey’s own research—published this month in the Annals of Internal Medicine—his team cautions: “widespread screening and treatment of subclinical thyroid dysfunction can also result in harms due to labeling, false-positive results, over-diagnosis and overtreatment.”
Hypothyroidism can have a number of causes, including autoimmune disease, radiation therapy, certain medications, even pregnancy and iodine deficiency. If you’re feeling tired for no reason or have any of the other signs or symptoms of hypothyroidism, such as dry skin, a pale, puffy face, constipation or a hoarse voice, you should see your doctor about testing.
If you’re older than 75, however, be aware that your TSH levels are naturally higher, so subclinical hypothyroidism may be trickier to diagnose. “Using age-adjusted TSH ranges is important to avoid over-diagnosis in the elderly,” Dr. Hennessey says, “as treatment of those without subclinical hypothyroidism is inappropriate, ineffective and potentially dangerous if over-dosage occurs.”