Thyroid disorder and diabetes
People with diabetes have an increased risk of developing thyroid disorder. In the general population, approximately 6% of people have some form of thyroid disorder. However, the prevalence of thyroid disorder increases to over 10% in people with diabetes.
Since people with one form of autoimmune disorder have an increased chance of developing other autoimmune disorders, people with Type 1 diabetes have a higher risk of autoimmune thyroid disorder. Up to 30% of women with Type 1 diabetes have some form of autoimmune thyroid disease. Postpartum thyroiditis, a form of autoimmune thyroid disease that causes thyroid dysfunction within a few months after delivery of a child, is three times more common in women with diabetes.
Although Type 2 diabetes is not an autoimmune disorder, there have been many reports showing a higher occurrence of thyroid diseases, particularly hypothyroidism, among people with Type 2 diabetes. The association between Type 2 diabetes and thyroid disorder, however, remains unexplained.
Effect on diabetes control
Since normal thyroid function is essential to regulate energy metabolism, abnormal thyroid function may have profound effects on blood glucose control in diabetes. Both hyperthyroidism and hypothyroidism can affect the course of diabetes, but their effects are somewhat different.
Hyperthyroidism. Hyperthyroidism is typically associated with worsening blood glucose control and increased insulin requirements. The excessive thyroid hormone causes increased glucose production in the liver, rapid absorption of glucose through the intestines, and increased insulin resistance (a condition in which the body does not use insulin efficiently). It may be important to consider underlying thyroid disorder if a person has unexplained weight loss, deterioration in blood glucose control, or increased insulin requirements. Sometimes hyperthyroidism may even unmask latent diabetes.
Having diabetes increases a person’s risk for heart disease, and many people with diabetes have a heart condition such as coronary heart disease or heart failure. Since hyperthyroidism causes rapid heart rate and increases the risk of abnormal heart rhythm, it may also bring on angina (chest pain), worsen heart failure or interfere with the treatment of heart failure, as well as further increase the risk of other heart problems.
Prolonged, untreated hyperthyroidism can cause excessive bone loss, leading to osteoporosis, or bone thinning. Osteoporosis raises the risk of bone fractures, making falling much more dangerous. People with diabetes who have peripheral neuropathy are at an increased risk for falls due to poor foot sensation and sometimes loss of proprioception, or loss of the stimuli that tell the brain where a body part is in space, in relation to other objects. Therefore, the combination of hyperthyroidism and diabetes, particularly when neuropathy is present, increases the risk of fractures that may result in disability, especially in the elderly.
Hypothyroidism. Hypothyroidism rarely causes significant changes in blood glucose control, although it can reduce the clearance of insulin from the bloodstream, so the dose of insulin may be reduced. More important, hypothyroidism is accompanied by a variety of abnormalities in blood lipid levels. This includes increased total cholesterol and LDL (low-density lipoprotein or “bad”) cholesterol levels, and increased triglyceride levels. The abnormal lipid pattern typical of Type 2 diabetes (low HDL, or “good” cholesterol; high triglycerides; and a high proportion of small, dense LDL particles) is usually worsened by hypothyroidism. These changes further raise the already high risk of cardiovascular diseases such as heart disease and stroke among people with diabetes.