Prevalence of abnormal thyrotropin concentrations measured by a sensitive assay in patients with type 2 diabetes mellitus.

The utilisation of assays for TSH with improved sensitivity has revealed that abnormal TSH results are frequently observed in patients with nonthyroidal illnesses, such as trauma, renal diseases, liver diseases or sepsis. The aim of this study was to investigate the prevalence of abnormal TSH concentrations, using a sensitive immunometric assay, in patients with type 2 (non-insulin-dependent) diabetes mellitus. The study population consisted of 290 type 2 diabetics, 159 females and 131 males aged 40 to 93 years (mean 60.6 +/- 11.9 years), hospitalised because of poor diabetic control or recent-onset diabetes (mean HbA1c value = 9.6 +/- 2.2%). All patients with TSH values outside the normal range (0.45 to 3.66 mlU/l) had FT4 assay and thyroid microsomal autoantibody assay performed on the same specimen of serum. Abnormal TSH concentrations were detected in 91 patients (31.4%). Subclinical hypothyroidism (high TSH, normal FT4) was most common (48.3%), followed by subclinical hyperthyroidism (low TSH, normal FT4) (24.2%) and by definite hypothyroidism (high TSH, low FT4) (23.1%). Definite hyperthyroidism (low TSH, raised FT4) was found in 4 patients (4.4%). None of the patients with low TSH values had increased FT3 concentrations. The prevalence of abnormal thyroid function test results was significantly higher in the female than in the male patients (40.9% vs. 19.8%, p < 0.0005) and in the insulin-treated patients than in those receiving oral hypoglycaemic agents (OHA) (37.3% vs. 23.1%, p < 0.02). Thirty patients with abnormal thyroid function test results (33.0%) had evidence of thyroid autoimmunity (titre of thyroid microsomal autoantibodies > 250 IU/l). Five thyroid microsomal antibody-negative patients had non-autoimmune thyroid diseases, 7 had nonthyroidal illnesses other than diabetes mellitus and 4 were receiving drugs known to affect the hypothalamic-pituitary-thyroid axis. Twenty-seven thyroid microsomal auto-antibody-negative patients with abnormal TSH values (17 with subclinical hypothyroidism and 10 with subclinical hyperthyroidism), who were not receiving drugs known to affect TSH secretion and were free of diseases other than diabetes mellitus, were retested after two months of adequate treatment of diabetes with OHA or insulin. TSH concentrations decreased in all but one patient with initial subclinical hypothyroidism and increased in all patients with initial subclinical hyperthyroidism. These changes were coupled with a significant fall of glycated haemoglobin values. In view of the transient changes in TSH secretion, we suggest that the diagnosis of thyroid dysfunction in type 2 diabetics should be delayed until improvement of the metabolic status.