[A case of thyroid hormone resistance syndrome due to previously undescribed mutation in the gene].

Thyroid hormone resistance (THR) syndrome is characterized by decreased sensitivity of peripheral tissues to active forms of thyroid hormones. We present a clinical case of a patient, demonstrating symptoms of hypothyroidism while having subnormal FT4 and normal TSH and total T3 levels. Massive parallel sequencing allowed us to identify a previously undescribed heterozygous THRA c.1198C>G (p.Leu400Val) variant that arose de novo. The results of genetic testing corresponded well with the patient's phenotype that made it possible to verify the diagnosis of THR. Levothyroxine replacement therapy did not resulted in a significant clinical improvement; the use of supraphysiological doses led to refinement of the lipid profile but was accompanied by the appearance of some symptoms of thyrotoxicosis.