Subclinical Graves' disease as a cause of subnormal TSH levels in euthyroid subjects

In order to elucidate causes of subclinical thyrotoxicosis, we reviewed records of thyroid function tests obtained in our hospital between 1990 and 1992 showing normal thyroid hormones and subnormal TSH levels, and analyzed underlying clinical conditions of the patients. Of 186 patients with normal T4 and/or free T4 and normal T3 and/or free T3 but subnormal TSH (< 0.1 mU/l) levels in serum, 150 were under treatment with antithyroid drugs for hyperthyroid Graves' disease or with thyroid hormones for hypothyroidism. Twelve were in remission after treatment for Graves' disease, and 4 had destructive thyroiditis. Of the remaining 20 patients, 4 had autonomously functioning thyroid nodule (AFTN), 9 had euthyroid ophthalmic Graves' disease (EOG), and 7 had diffuse goiter without apparent ophthalmopathy (DG). When thyroid stimulating antibodies (TSAb) were measured in the last 3 groups of the patients, they were detected in none with AFTN but in all patients with EOG and DG. These 7 DG patients without ophthalmopathy had a clinical feature showing unstable thyroid functions, changeable to euthyroidism, overt hyperthyroidism and even hypothyroidism during follow-up. In conclusion, TSAb measurement is useful for detection of subclinical Graves' disease in euthyroid subjects with subnormal TSH levels in serum.