The thyroid gland, a small organ in the neck, produces hormones that regulate the body’s metabolism and energy use. Diabetes mellitus is a condition where the body cannot properly regulate blood sugar (glucose) due to issues with the hormone insulin. Although they affect different systems, thyroid dysfunction has a significant relationship with the development or management of diabetes. Thyroid disease directly impacts blood sugar stability, making the treatment of one condition relevant to the other.
The Autoimmune Connection Between Thyroid Disease and Diabetes
The connection between thyroid disease and diabetes is rooted in the body’s immune system, which can mistakenly attack both the pancreas and the thyroid gland. Type 1 diabetes is an autoimmune disease resulting in insulin deficiency. Similarly, the most common thyroid conditions—Hashimoto’s thyroiditis (hypothyroidism) and Graves’ disease (hyperthyroidism)—are also autoimmune disorders.
This shared vulnerability means that a person with one autoimmune condition is more likely to develop another. Approximately 17% to 30% of adults with Type 1 diabetes also have an autoimmune thyroid disorder. Hashimoto’s thyroiditis is the most frequent co-occurring condition found alongside Type 1 diabetes. These conditions often cluster together due to a broader genetic predisposition, sometimes referred to as Autoimmune Polyglandular Syndrome.
How Thyroid Hormone Imbalance Affects Blood Sugar Control
Beyond the shared autoimmune origin, thyroid hormones are metabolic regulators that exert a powerful influence on glucose metabolism. Any imbalance, whether too high or too low, disrupts blood sugar control. This physiological connection is particularly relevant to Type 2 diabetes and the difficulty in managing blood sugar.
In hyperthyroidism, the body’s metabolism speeds up significantly. Excess thyroid hormones increase glucose production by the liver. They also accelerate glucose absorption from the gastrointestinal tract and increase the rate at which insulin is cleared from the bloodstream. This combination leads to insulin resistance and hyperglycemia, often requiring a higher dose of diabetes medication.
Conversely, hypothyroidism slows down the body’s metabolic functions. This slower pace reduces the liver’s glucose production and also prolongs the half-life of insulin because insulin clearance is decreased. If a person with diabetes is taking insulin or certain oral medications, this slowed metabolism can cause the medication to remain active longer than expected. The slower clearance rate can lead to an increased risk of hypoglycemia if medication dosages are not appropriately decreased. Subtle elevations in thyroid-stimulating hormone (TSH) levels have been associated with increased insulin resistance.
Treating Both Conditions Together
The treatment of one condition is inseparable from the management of the other. For individuals with Type 1 diabetes, medical guidelines strongly recommend regular screening for autoimmune thyroid disease. Initial screening for thyroid-stimulating hormone and thyroid antibodies is often performed at the time of diabetes diagnosis, with follow-up testing recommended every one to two years.
Treating the underlying thyroid dysfunction can significantly improve glycemic control. For example, treating hypothyroidism with synthetic hormone replacement, such as levothyroxine, helps the body’s metabolism normalize. Similarly, controlling an overactive thyroid decreases excessive glucose production and reduces insulin resistance.
Medical professionals must monitor a patient’s diabetes medications as thyroid treatment progresses. As thyroid function returns to a normal range, the patient’s insulin requirements may change. A patient treated for hyperthyroidism may see their insulin needs decrease as the condition stabilizes. Conversely, a patient starting treatment for hypothyroidism may see their needs increase. This integrated approach requires close communication among the healthcare team to prevent episodes of severe hyper- or hypoglycemia.