Diabetes mellitus not infrequently coexists with hypo- and hyperthyroidism. Hyperthyroidism aggravates glucose intolerance. A review of this phenomenon reveals multiple mechanisms, which include increased hexose intestinal absorption, decreased responsiveness to insulin, and increased glucose production. Conflicting results are obtained when circulating insulin level is measured in thyrotoxicosis. The role of glucagon and a-cell sensitivity is unclear. Diabetes mellitus influences the assessment of thyrotoxicosis by falsely decreasing the blood levels of thyroxine (T4) and triiodothyronine (T3) during severely uncontrolled hyperglycemia. Hypothyroidism is found in about 3% of patients with insulin-dependent diabetes mellitus (IDDM). Moreover, 13–20% of IDDM patients have elevated blood thyrotropin levels and anti-thyroid antibodies. Hypothyroidism per se seems to ameliorate hyperglycemia. A subtype of IDDM shares similar immunogenetic features with familial autoimmune thyroiditis. Studies of IDDM probands who show a high prevalence of circulating thyroid antibodies reveal the presence of such antibodies in their first-degree relatives. Circulating islet-cell antibodies, detected in a majority of IDDM patients at the onset of their disease, tend to persist only in those patients with coexistent polyendocrine autoimmune disease, including thyroiditis. Similar human leukocyte antigen (HLA) locus types are associated with thyroiditis and IDDM, namely HLA-Dr3 and -Dr4.

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