Thyroid Disease and the Risk of Type 2 Diabetes

Woman testing for diabetes
BSIP/UIG / Getty Images

As part of the endocrine system, your thyroid plays an important role in helping to regulate and maintain balance in your blood glucose level. There is some evidence of a link between thyroid levels and the risk of type 2 diabetes. Korean researchers have done a more in-depth study to evaluate the rate of type 2 diabetes as it relates to baseline thyroid stimulating hormone (TSH) levels as well as changes in TSH, free thyroxine (Free T4), and free triiodothyronine (Free T3).

Breaking Down the Study Design

We know that overt hypothyroidism (TSH levels above 10), as well as subclinical hypothyroidism, are associated with hyperglycemia (elevated blood sugar) as well as increased insulin resistance.

The researchers evaluated more than 6,200 people who had normal—known as “euthyroid”—thyroid function, over a six-year period. Among that group, 229 people developed type 2 diabetes. Type 2 diabetes was diagnosed as fasting glucose above 126 mg/dL and/or a hemoglobin A1C level above 6.5 percent.

The researchers adjusted for hemoglobin A1C (sometimes referred to as A1C) listings, as well as fasting glucose, and determined the following key finding:

Those patients who showed increases in their TSH level, even within the reference range, had a greater risk of developing type 2 diabetes, as compared to those who did not have elevations in TSH. 

The participants were split into two groups, divided according to the numbers of metabolic risk factors they had at the start of the research. The metabolic risk factors included:

Group 1 (the low-risk group) had fewer than two metabolic risk factors, and the high-risk group 2 had two or more metabolic risk factors.

The participants were subdivided as “improved,” “stable,” or “ aggravated” based on the changes in their risk factors. Those who had no change in risk factors were classified as stable.

What the Research Found

Among the women studied who developed type 2 diabetes, they had a significantly higher baseline TSH, though still within the reference range. According to researchers, both men and women had the same results: TSH levels that increased over time, within the reference range, increased the type 2 diabetes risk, while higher T3 and free fT4 decreased the type 2 diabetes risk. In all groups, an increase in TSH levels, with a decrease in T3 and free T4, was linked to an increased incidence of type 2 diabetes.

The results of the study suggest that changes taking place in the TSH and thyroid hormone levels prior to a diagnosis of type 2 diabetes are a factor in the risk of developing the condition. A pattern of gradually increasing TSH, along with decreases in T3 and free T4, were associated with increased risk of type 2 diabetes. This risk was evident in the general population and was not dependent on gender and autoimmune thyroid status.

Interestingly, while the changes in thyroid hormones are not better predictive factors than fasting glucose or HbA1c testing, the decline in free T4 and increase in TSH were considered to be better predictors of type 2 diabetes risk than BMI or changes to the BMI. Overall, the study concluded that subtle changes in thyroid hormones, which in the past have not been recognized as a risk factor, appear to be an additional risk for type 2 diabetes, even in people who have otherwise “normal” thyroid levels and no preexisting thyroid disease or thyroid autoimmunity.

How Glucose and Insulin Work

It’s important to understand how glucose and insulin work.

Glucose—a type of sugar—provides energy to your cells. The glucose comes from your food and from your liver, which makes and stores glucose. Typically, when your glucose levels drop, your liver takes a stored form of glucose—glycogen—and breaks it down to keep delivering a steady stream of glucose.

Insulin is a hormone produced by your pancreas. The pancreas—along with the thyroid—is part of your endocrine system. Insulin draws the glucose out of your bloodstream and helps it move into your cells to deliver energy. As blood sugar drops, so does the level of insulin your pancreas is secreting.

If you have type 2 diabetes, as glucose builds up in your bloodstream, either your pancreas is unable to release enough insulin, or your cells don't respond to the insulin, known as insulin resistance. If you do not have a diagnosed thyroid condition, but have had increasingly higher TSH test results—even if they fall within the reference range—you may want to have periodic evaluation of your risk for type 2 diabetes.

Type 2 Diabetes Risk Factors and Symptoms

In addition to the increases in TSH levels, other risk factors for type 2 diabetes include:

  • Being overweight
  • A higher percentage of abdominal fat
  • Lack of activity and exercise
  • A personal or family history of type 2 diabetes
  • Being African-American, Hispanic, Native American, or Asian-American
  • Age: the risk goes up after age 45
  • A history of prediabetes (elevated blood sugar)
  • A history of gestational diabetes
  • A history of polycystic ovarian syndrome (PCOs)

You should also be knowledgeable about the symptoms and early-warning signs that you may have developed type 2 diabetes, which can include:

  • Increased thirst
  • Dry mouth
  • Frequent urination
  • Increased hunger, especially after eating
  • Fatigue and exhaustion
  • Numbness and tingling in your hands and feet
  • Pain in your calves
  • Slow healing of wounds and cuts
  • Blurred vision
  • Irritability, depression
  • Unexplained weight loss or weight gain
  • Dry, itchy skin
  • More frequent vaginal yeast infections
  • Erectile dysfunction (impotence)

Diagnosing and Treating Type 2 Diabetes

Your doctor may do periodic testing to evaluate for an diagnose type 2 diabetes. The tests include:

The treatment for type 2 diabetes focuses on lowering blood sugar, typically through a combination of medications that help improve the body’s sensitivity to insulin, as well as dietary changes, exercise, and lifestyle changes that can help lower the overall levels of glucose in the bloodstream. If you've been diagnosed, you may be surprised and shocked. Work with your doctor to get your numbers under control and best take care of your health.

View Article Sources
  • Fun, J. Jee, H. et al. “Association Between Changes in Thyroid Hormones and Incident Type 2 Diabetes: A Seven-Year Longitudinal Study,” Thyroid. January 2017, 27(1): 29-38. doi:10.1089/thy.2016.0171.