Low-Normal Thyroid Levels Associated With Metabolic Syndrome

Overweight women at nutritionist's office

A 2017 study has shown that variations of thyroid hormone within the normal reference range may be associated with an increased risk of metabolic syndrome. These findings have important implications for the health of people with hypothyroidism.

What Is Metabolic Syndrome?

Metabolic syndrome refers to a group of conditions or risk factors that, when found together, increase your risk for heart disease, stroke, and type 2 diabetes. There are five key signs that point to metabolic syndrome. A diagnosis of metabolic syndrome is made when you have at least three of the metabolic risk factors, which include:

  • A large waistline. A large waist is also referred to as abdominal obesity. According to the National Institutes of Health, this is defined as a waistline that measures more than 35 inches (89 centimeters) for women and 40 inches (102 centimeters) for men. A large waistline increases your risk of heart disease.
  • High triglyceride levels, or needing medication for high triglycerides. Triglycerides are a type of fat in your bloodstream, and they are usually measured along with your cholesterol levels. Normal triglyceride levels are less than 150. Levels of 150 to 199 are considered borderline high, and anything over 200 is considered a high level. High triglycerides increase your risk of heart disease.
  • Low HDL cholesterol level, or needing medication to lower HDL cholesterol. HDL stands for high-density lipoprotein, and this type of cholesterol is known as the "good" cholesterol because it helps clear other types of cholesterol out of your arteries. A low HDL cholesterol increases your risk of heart disease.
  • High blood pressure, or needing medication to lower blood pressure. Blood pressure measures how hard your blood pushes against your artery walls as it’s pumped by your heart. Blood pressure readings have two numbers, for example, 130/80 mmHg. The top number is called the systolic blood pressure and measures the highest pressure when your heart is beating. The bottom one is called the diastolic blood pressure and measures the lowest pressure between heartbeats. High blood pressure is defined as a level above 140/90 mmHg. High blood pressure increases your risk of heart disease and stroke.
  • High fasting glucose (blood sugar), or needing medication to lower your blood sugar. High blood sugar is defined as being above 130 mg/dL (milligrams per deciliter) 8 hours after not eating or drinking. High blood sugar increases your risk of heart disease, and can also be a precursor to type 2 diabetes.

One of the key triggers for metabolic syndrome is a condition known as insulin resistance. In insulin resistance, your body becomes less sensitive to insulin. The job of insulin is to help move blood sugar out of the bloodstream and into your cells. When you are resistant to insulin, you can develop chronically high blood sugar levels. This can cause weight gain and puts you at a higher risk of developing type 2 diabetes.

Other contributing factors to developing metabolic syndrome include obesity, lack of exercise and physical activity, increasing age, and chronic stress.

Hypothyroidism and Metabolic Syndrome

Thyroid hormone is crucial to the body’s ability to metabolize fats and glucose, and to maintaining a healthy blood pressure. When you are hypothyroid and do not have enough circulating thyroid hormone, these functions can be impaired. The study, which was published in the July 2017 issue of the journal Thyroid, evaluated more than 2,000 people, measuring their levels of fasting glucose, insulin, free thyroxine (free T4), and thyroid stimulating hormone (TSH) levels at the start of the study, and during three follow-up studies at 3-year intervals. Overall, the study looked at data over a 10-year period.

The study found that in those people in the study who were not obese, and who had a TSH and free T4 levels within the reference range, the free T4 level was still a significant predictor for metabolic syndrome. Lower free T4 levels—within the reference range—were associated with a significantly increased risk of developing metabolic syndrome. These findings were independent of age, sex, or pre-existing insulin resistance.

Some other important findings:

  • Low free T4 levels within the reference range were associated with an increased body mass index (BMI).
  • Low free T4 levels within the reference range were associated with an increased risk of high triglyceride levels.
  • Low free T4 levels within the reference range were associated with an increased risk of high blood pressure.
  • Low free T4 levels within the reference range were associated with an increased risk of abdominal obesity.

The researchers concluded that free T4 levels in the lower end of the reference range are associated with increased risk of metabolic syndrome. They suggested that “detection of low-normal free T4 levels could indicate metabolically unhealthy subjects with combined risk factors who could benefit from early screening and medical intervention.”

The researchers also recommend that further research should be conducted to determine if maintaining free T4 levels in the upper end of the reference range could help reduce insulin resistance and reduce the risk of developing metabolic syndrome in people who otherwise do not have a thyroid condition.

A Word From Verywell

If you have low free T4 levels, you may want to talk to your practitioner about increasing your dosage of thyroid hormone replacement medication to optimize your thyroid treatment and reduce your risk of developing complications like metabolic syndrome.

If you are at risk of metabolic syndrome or have already been diagnosed, there are some key ways to help reverse it:

  1. Exercise. Exercise helps you reduce abdominal obesity, lower blood pressure, and glucose levels, and may help you lose weight.
  2. Eat a healthy diet. Many doctors suggest you follow a heart-healthy Mediterranean-type diet that includes "good" fats (like monounsaturated fats in olive oil and avocados), lean proteins, fruits, and vegetables. You should also avoid sugar, refined carbohydrates, artificial sweeteners, and limit alcohol intake.
  3. Lose weight. This may require a change to what and how much you eat and an increase in your level of exercise.
  4. Quit smoking. Smoking can increase the risk of heart disease.
  5. Get treatment for elevated blood sugar. Drugs that improve your insulin sensitivity—such as metformin (Glucophage) or injectable drugs like Byetta and Symlin—may help reverse insulin resistance and prevent the development of type 2 diabetes.
View Article Sources
  • Ladan, M. et. al. “Variations in Serum Free Thyroxine Concentration Within the Reference Range Predicts the Incidence of Metabolic Syndrome in Non-Obese Adults: A Cohort Study.” Thyroid. July 2017, 27(7): 886-893.