Diagnosis of hypothyroidism requires documentation of low levels of thyroid hormones (i.e. T4 and T3). However, the diagnosis is refined by also measuring levels of thyroid-stimulating hormone (TSH), which is produced in the brain and acts on the thyroid gland to stimulate T4 and T3 production. If TSH is high, then thyroid gland hormone secretion is the primary defect. However, if TSH is low, then the defective supply of this hormone from the brain (pituitary gland) is the cause of the hypothyroidism.

Around 50–60% of all patients with autoimmune hyperthyroidism will go on to develop systemic disease, involving their skin, eyes, heart, etc. In a large, cross-sectional multicentre study, 3286 Caucasian subjects (2791 with Graves’ disease, 495 with Hashimoto’s thyroiditis) attending UK hospital thyroid clinics were investigated for coexisting autoimmune disorders. The frequency of a further autoimmune disorder in these patients was investigated by questionnaire, and revealed a rate of 9.67% for those with Graves’ disease and 14.3% for those with Hashimoto’s thyroiditis. The most common coexisting autoimmune condition was rheumatoid arthritis, but other diseases were also present, including pernicious anaemia, systemic lupus erythematosus (SLE), Addison’s disease, celiac disease and vitiligo. Thyroid diseases are frequent in patients with SLE, particularly hypothyroidism and thyroid autoantibodies.