Subclinical Hypothyroidism and Metabolic Syndrome

vial of blood and paper marked for TSH test
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According to the National Heart Lung and Blood Institute, about 47 million adults in the United States (almost 25 percent of the population) have metabolic syndrome, a combination of risk conditions that increase the risk of heart disease, diabetes, and stroke. The number of people suffering from metabolic syndrome also is on the rise.

Now, researchers have discovered that low-normal thyroid function -- a condition known as subclinical hypothyroidism -- is a risk factor for metabolic syndrome.

What Is Metabolic Syndrome?

Metabolic syndrome is the name given to a syndrome that is seen more often in people who are overweight or obese, and who have specific conditions that increase the risk of heart disease, diabetes, stroke, and other health problems. Metabolic syndrome is diagnosed when three out of the following five risk factors are evident:

A Large Waistline

This may be referred to as abdominal obesity, or being "apple-shaped" or having a "Buddha belly." But according to the National Cholesterol Education Program (NCEP) and the American Heart Association, the criteria for "large waistline" is measuring over 35 inches for women and over 40 inches for men. For people who are genetically predisposed to diabetes, the limits are even lower, at 31 to 35 inches for women and 37 to 39 inches for men.

Elevated Triglycerides

Triglycerides are a type of fat that circulates in the bloodstream and increases the risk of heart disease.

An elevated level is considered to be 150 milligrams per deciliter (mg/dL) or higher.


High-density lipoprotein (HDL) is a type of cholesterol often referred to as the "good" cholesterol because it helps lower your risk of heart disease. A level of less than 40 mg/dL in men or less than 50 mg/dL in women is considered low.

Elevated Blood Pressure

Blood pressure is recorded as two numbers, written one on top of or before the other, such as 120/80, or referred to as "120 over 80." The top number is the systolic blood pressure, and it's considered elevated if it goes above 130 and the bottom number - the diastolic - is considered elevated if it is above 85.

Elevated Fasting Blood Sugar

Fasting blood sugar, also known as fasting glucose, can start to elevate as a warning sign of diabetes. A level of 100 mg/dL or higher is considered elevated.

As noted, at least three of the risk factors above qualify a person for a diagnosis of metabolic syndrome. But the more risk factors a person has, the greater the risk of heart disease, diabetes, or stroke. According to the NCEP, a person with metabolic syndrome is twice as likely to develop heart disease and five times as likely to develop diabetes as someone without metabolic syndrome.

Metabolic syndrome typically develops in people who are overweight or obese, who don't engage in physical activity, or who have a diet that promotes insulin resistance. Family history and age are also underlying causes.

The Thyroid Connection

Researchers have now found that even subtle changes in thyroid function increase the risk for metabolic syndrome.

The link between overt hypothyroidism and an increased risk of heart disease has already been established. But research published in the February 2007 issue of the Journal of Clinical Endocrinology and Metabolism found a connection between thyroid function and metabolic syndrome in people who have normal thyroid stimulating hormone (TSH) levels.

What the researchers found was that in those with normal TSH levels, the thyroid hormone level known as free T4 was important. Free T4 levels that were slightly low, but still within the normal range, significantly increased the risk of many risk factors for metabolic syndrome.

Lower levels of another thyroid hormone, free T3, were linked to risk factors including higher total cholesterol, LDL cholesterol, and triglycerides.

The researchers concluded that for people who have normal TSH levels, even slight changes in free T4 and free T3 levels can have an effect on the risk of metabolic syndrome and heart disease.

The researchers recommend that a study be done to determine whether early treatment of thyroid dysfunction might reduce the risk.

What This Means for You

If future research does find that early treatment helps, free T4 and free T3, and not just TSH, will become key measurements in thyroid diagnosis and treatment decisions.

This research also suggests that if you are being treated for hypothyroidism, you should be monitored for signs of metabolic syndrome. If you are hypothyroid, you'll want to make every effort to reduce your metabolic syndrome risk factors.

Addressing the risk factors can be complex, but generally, involves a combination of any or all of the following approaches:

  • Overall efforts to lose weight, including diet and exercise
  • Specialized dietary changes to combat insulin resistance and improve cholesterol levels
  • Exercise
  • Medications to lower triglycerides, raise HDL, lower blood pressure, lower/manage blood sugar
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View Article Sources
  • Roos, Annemieke, et. al. "Thyroid Function Is Associated with Components of the Metabolic Syndrome in Euthyroid Subjects," The Journal of Clinical Endocrinology & Metabolism Vol. 92, No. 2 491-496,
  • Mayo Clinic, "Metabolic Syndrome," Mayo Foundation for Medical Education and Research, 1998-2007
  • National Heart Lung and Blood Institute, "What is Metabolic Syndrome," April 2007,