People with thyroid cancer treated with thyroidectomy have as much as a 40% increased risk of developing type 2 diabetes, regardless of their age, with the elevated risk observed with low as well as high doses of postoperative levothyroxine, new research shows.
“This is the first population-based study to demonstrate an elevated risk of type 2 diabetes in postthyroidectomy patients with thyroid cancer, compared with that in matched controls,” wrote the authors of the research, published recently in the Journal of Clinical Endocrinology & Metabolism.
“Notably, there was a U-shaped relationship between postoperative levothyroxine dosage, a surrogate marker of TSH suppression, and the risk of type 2 diabetes,” said Hye Jin Yoo, MD, of the division of endocrinology and metabolism, Korea University College of Medicine, Seoul, and colleagues.
While other studies have linked thyroidectomy for thyroid cancer with an elevated risk for other metabolic conditions, including coronary heart disease and ischemic stroke, the relatively high diabetes risk is unexpected, said Tyler Drake, MD, an endocrinologist with the Minneapolis VA Health Care System.
“A 40% increased risk of diabetes is a big surprise,” he said in an interview.
“Diabetes is very common, with about one in 10 U.S. adults having type 2 diabetes, but a 40% increased risk in thyroid cancer patients is higher than I see in my clinical practice. [However], it is important to note that the [highest] risk was predominantly among the groups on the lowest and highest doses of levothyroxine,” said Dr. Drake, assistant professor of medicine at the University of Minnesota, Minneapolis.
U-shaped relationship between levothyroxine dose and diabetes risk
The findings are from a study of 36,377 patients with thyroid cancer in the National Health Insurance Service (NHIS) database in Korea who had undergone a thyroidectomy between 2004 and 2013.
The patients were matched 1:1 with controls who had nonthyroid cancers. Their mean age was 46.6 years, about 30% were male, and their mean body mass index was 23.8 kg/m2.
Over a mean follow-up of 6.6 years, the patients with thyroid cancer had a significantly higher risk of developing type 2 diabetes, at a rate of 47.5% (10,812) compared with 36.9% (9414; HR, 1.43; P < .001) in the control group, after adjustment for factors such as age, sex, BMI, smoking, drinking, systolic blood pressure, and fasting glucose.
The risk of type 2 diabetes among those with thyroid cancer was higher among the 83.2% of patients who underwent a total thyroidectomy compared with the 16.8% who had a unilateral lobectomy (HR, 1.06; P < .001).
In addition, those with thyroid cancer who received the lowest as well as highest dosages of levothyroxine had significantly higher risks of type 2 diabetes compared with controls (HR, 1.50 and 1.39, respectively; both P < .001).
A closer look at quartiles of levothyroxine dosing showed the first (lowest) quartile (defined as a mean levothyroxine dosage of < 101 mcg/day) was associated with an increased risk of type 2 diabetes compared with the second quartile group (101-127 mcg/day; HR, 1.45), as was the fourth quartile (≥ 150 mcg/day; HR, 1.37), while a decreased risk of type 2 diabetes was observed in the third quartile group (128-149 mcg/day versus the second quartile group; HR, 0.91).
“This result suggests a U-shaped relationship between the mean levothyroxine dosage and risk of type 2 diabetes in postthyroidectomy patients with thyroid cancer,” the authors said.
However, “consistent with previous studies, the present study showed that the highest risk of type 2 diabetes was observed in patients with thyroid cancer who were treated with the lowest mean dosage of levothyroxine,” they noted.
“This result suggests that inadequate supplementation of thyroid hormones may worsen glucose metabolism and should therefore be avoided.”