[Evaluation of TSH suppression using a 3d-generation TSH assay: diagnostic and therapeutic consequences]

We measured basal and TRH-stimulated TSH values with a 3rd generation assay in patients under suppressive thyroxine (T4) therapy for thyroid carcinoma or goiter and in patients with overt hyperthyroidism. All hyperthyroid patients had undetectable basal TSH levels (< 0.01 mU/l). In patients on suppressive T4 treatment basal TSH values were undetectable in 37.5% (= group A) and measurable in the intermediate range of 0.01-0.05 mU/l in 62.5% (= group B). After TRH administration there was no TSH increase in the hyperthyroid patients (< 0.01 mU/l in 100%). T4-treated patients of group A showed no relevant TSH stimulation in 58% (peak < or = 0.05 mU/l = total TSH suppression). An increase of > 0.05 mU/l could be measured in 42% of group A and 100% of group B (= subtotal TSH suppression). Many of these TSH values measured by a 3rd generation assay are in the undetectable range for most of the commercially available 2nd generation assays (< 0.10 mU/l).

TSH assay of the 3rd generation decisively improves the diagnosis of hyperthyroidism and probably also borderline hyperthyroidism. Against the background of increased risk of osteoporosis and cardiac function disorders as a result of suppressive T4 treatment, our results suggest the following practical therapeutic guidelines: total TSH suppression with the risk of iatrogenic hyperthyroidism should be confined to high risk patients (eg with metastatic thyroid cancer). Where the risk is low, however (thyroid cancer with favourable prognosis, goiter, goiter prophylaxis), only subtotal TSH suppression is indicated.