Background. The elevation of thyroid hormone with a normal or elevated TSH occurs uncommonly. This has different causes and pose a diagnosis challenge namely between TSH-secreting pituitary adenoma (TSHoma) and resistance to thyroid hormone. The accurate diagnosis is essential, because delayed diagnosis of TSHoma can lead to tumour growth and poor surgical cure rates, whereas medical, surgical or radioablative treatments in patients with resistance to thyroid hormone are usually unnecessary and potentially harmful. Case Report. A 23-years-old women with palpitations, fatigue, insomnia and exophthalmia with elevated serum free T4 and TSH, medicated with methimazole 5mg 3id was sent to evaluation in endocrinology department. She did not report headaches or visual problems. Patient’s laboratory tests at admission: TSH 9.6 µUI/mL (0.4—4.0 µUI/mL), Free T4 2.1 pg/dL (0.8—1.9 pg/dL). After stopping anti-thyroid drug presented TSH 2,9 µUI/mL (0.4—4.0 µUI/mL), Free T4 3.7 pg/dL (0.8—1.9 pg/dL), Free T3 11 pg/mL (1.8—4.2 pg/mL); antithyroid peroxidase and antithyroglobulin antibodies and thyroid stimulating immunoglobulin were undetectable; thyroid ultrasound revealed small heterogeneous goiter; thyroid technetium scintigraphy showed diffuse glandular hyperfunctioning; Magnetic resonance imaging revealed a microadenoma with 7.5 mm in the left side of pituitary. Remaining anterior pituitary hormones were within normal ranges. The thyrotropin-releasing hormone stimulation test was performed and revealed TSH at 0’ 1.7 µUI/mL, 20’ 14 µUI/mL and 60’ 11 µUI/mL, with free T4 2.3 pg/dL and free T3 5.9 pg/mL, which was consistent with thyroid hormone resistance syndrome. In this clinical setting genetic test was performed and revealed mutation in heterozygosity in THRβ gene: c.1030G>A, p.Gly344Arg. Patient’s mother was also tested and no mutation was found. Her father was not available to do the genetic test. No pituitary surgery or thyroidectomy was performed, nor were prescribed any anti-thyroid drugs. Conclusions. In this case, an innapropriate TSH secretion was identified and the clinical, biochemical and genetic investigations were consistent with resistance to thyroid hormone. Known that as many as 15% non-ill people may have a small, nonfunctioning pituitary adenoma, patients with thyroid hormone resistance may have incidentally abnormal imaging findings. The high level of clinical suspicion and the proper laboratory, genetic and radiological studies, conduct to a correct diagnosis and prevent unnecessary and potential harmful therapies.