In this case report, an 18 years old female subject with incidentally detected thyroid stimulating hormon (TSH) elevation due to macro-TSH without clinical findings of thyroid disorder was defined. Initially, the laboratory investigations revealed high TSH [11.45 µIU/mL; normal reference interval (NRI: 0.50-5.50 µIU/mL)] with electrochemiluminescence immunoassay (ECLIA) method. When L-thyroxine and L-triodothyronine treatments were found to be ineffective for lowering TSH, polyethylene glycol (PEG) method for TSH measurement was planned to rule out the posibility of macro-TSH. TSH level upon using the PEG method was found to be within normal ranges as 1.96 µIU/mL. In conclusion, normal free thyroxine level accompanied by elevated TSH is mostly encountered in cases of subclinical hypothyroidism. Such a laboratory finding can also be caused by macro-TSH which is a rare condition. To avoid unnecessary investigations and treatment, macro-TSH should be kept in mind in patients with high TSH levels without symptoms of hypothyroidism in the differential diagnosis.
Subclinical hypothyroidism, macro-TSH, polyethylene glycol