Potential mechanisms
Abnormal thyroid function, including hypo- and hyperthyroidism, following thyroidectomy and subsequent treatment with levothyroxine, is known to have potentially detrimental effects on glucose regulation among patients with thyroid cancer.
The potential mechanisms linking hypothyroidism with diabetes specifically include the possibility that insulin becomes unable to promote the utilization of glucose by muscles and adipose tissue. However, thyroid hormone replacement has been associated with a normalization of insulin sensitivity, the authors noted.
Meanwhile, glucose intolerance is common among patients with hyperthyroidism, largely due to an increase in hepatic glucose production, and likewise, the normalization of thyroid levels among those treated with methimazole has been linked to normalization of glucose and lipid metabolism alterations.
Dr. Drake noted that an important study limitation is that patients were analyzed based on their levothyroxine dose and not their TSH values, which the authors explain was due to the unavailability of the TSH values.
“By looking at levothyroxine doses, and not TSH values, it is possible some patients were being improperly treated with either too much or too little levothyroxine,” Dr. Drake noted.
Control group should have had hypothyroidism
The findings nevertheless shed light on the risk of diabetes following thyroidectomy for thyroid cancer, Anupam Kotwal, MD, commented on the study.
“This study is significant because it addresses an important topic exploring the link between thyroid dysfunction and metabolic disease, in this case ... hypothyroidism, due to surgery for thyroid cancer and type 2 diabetes,” Dr. Kotwal, assistant professor of medicine in the division of diabetes, endocrinology & metabolism at the University of Nebraska Medical Center, Omaha, said in an interview.
In terms of other limitations, Dr. Kotwal noted that the controls did not have hypothyroidism; therefore, “from this study, it is impossible to confirm whether hypothyroidism from any cause would be associated with higher incidence of diabetes or if it is specific to thyroid surgery for thyroid cancer.
“It would have been useful to have a control group of autoimmune primary hypothyroidism to evaluate the rate of diabetes during a similar follow-up duration,” Dr. Kotwal said.
“Hence, cohort studies with more granular data such as degree of TSH suppression and having a control group of hypothyroid patients due to autoimmune thyroid disease are needed to better understand this risk.”
Dr. Kotwal and Dr. Drake have reported no relevant financial relationships.
A version of this article first appeared on Medscape.com.