<h4>Background</h4>Measuring thyroid hormone levels is essential and helpful in the diagnosis of thyroid diseases.<h4>Case presentation</h4>We had a patient with undetectable serum TSH level by the Siemens immunoassay, whereas by ELISA and Roche-Abott immunoassay, serum level of TSH was in the normal range.<h4>Conclusion</h4>If the result of TSH level revealed very low or undetectable through one specific method, it should not be considered necessarily as a symptom of hyperthyroidism. Here, TG is linked to hyperthyroidism.