Hypothyroidism is the most common type of thyroid disorder, and is the situation where the thyroid fails to make enough thyroid hormone to regulate the body's metabolism. The most common form of hypothyroidism is autoimmune, and is called Hashimoto's disease. With this disease, the body's immune system (which normally protects you from disease) thinks the thyroid is a foreign invader and tries to destroy the thyroid. When this damage is done to the thyroid, it can become larger in size (and is called a goiter) as well as underactive in its functioning.
Symptoms of underactive thyroid gland functioning, no matter what the cause, include fatigue, dry, coarse skin and hair, inability to tolerate cold weather, weight gain, hoarse voice, and heavy or irregular menstrual periods. The symptoms develop so slowly that sometimes people just think they are growing old prematurely. Lab tests for hypothyroidism are done using a standard blood sample, and include measurement of the TSH (thyroid stimulating hormone) levels, thyroid hormone levels (called T3 and T4), and sometimes thyroid antibodies. In a patient with the symptoms, an elevated TSH level with simultaneous low levels of T3 and T4 are conclusive evidence that the thyroid gland is underactive. Indeed, a high level of TSH with normal levels of T3 and T4 are considered very suggestive evidence of impending hypothyroidism and worthy of treatment.
By the way, note the paradox: the TSH level is high in most underactive thyroid disorders. This is easy to understand when it is understood that TSH is made elsewhere, in the pituitary gland, and if the thyroid is failing to make its thyroid hormone, then the pituitary gland attempts to stimulate the thyroid to make more thyroid hormone, hence the TSH level goes up. For people without symptoms of hypothyroidism, measurement of the TSH level can be used as a screening test to look for early thyroid gland problems; sometime, it's also recommended that thyroid antibodies be part of the screening process.
It's been known for years that diabetes and thyroid problems can go hand-in-hand. More specifically, type 1 diabetes (T1DM), which is an autoimmune disorder, is associated with other autoimmune disorders including autoimmune thyroid disorders. These autoimmune thyroid disorders may cause the thyroid to become hypothyroid, or, sometimes, to become overactive (hyperthyroid).
But the evidence that the more common type 2 diabetes (T2DM) might be associated with hypothyroidism has been less persuasive until recently. An abstract presented at a recent scientific meeting, Type 2 Diabetes Mellitus and Primary Hypothyroidism Should TSH Be Tested in All Patients With Diabetes? makes the case that patients with T2DM have an increased risk of hypothyroidism. In the study, 1,848 adults with T2DM were compared with 3,313 individuals without diabetes, and it was found that the prevalence of hypothyroidism with T2DM was 5.7% compared with 1.8% in the non-diabetes group. The authors recommend that all patients with T2DM be screened for thyroid disease.
Treatment for hypothyroidism is straight-forward: give the patient thyroid hormone replacement. How much thyroid hormone to give is about the only question: give too little, and the symptoms continue; give too much and the patient can become hyperthyroid. Physicians adjust the dose of thyroid hormone replacement by rechecking the TSH level rather than judging by symptom levels: when the TSH is in the normal range, it can be assumed that the amount of thyroid hormone replacement therapy is appropriate.
The American Diabetes Association suggests for patients with T1DM that "TSH concentrations should be measured after metabolic control has been established. If normal, they should be re-checked every 1-2 years, or if the patient develops symptoms of thyroid dysfunction..." But there's no ADA recommendation for people with T2DM. The authors of the abstract advised testing "similar to what occurs in type 1 diabetes."
Testing for hypothyroidism is easy to do, thyroid disease is common in the age group that has T2DM, treatment for hypothyroidism is easy and rewarding in relieving symptoms and decreasing the risk of future disease. If you have diabetes, whether T1DM or T2DM, it seems reasonable that you should have your TSH level checked every year or two.