Resistance to thyroid hormone (RTH) is characterized by decreased tissue responsiveness to thyroid hormone, due mainly to mutation of the thyroid hormone receptor (TR) beta gene. It has been reported that serum of patients with RTH lacks autoantibodies against thyroglobulin (Tg) and thyroid peroxidase (TPO), except in rare cases where there is co-occurrence of coincidental autoimmune thyroiditis. Here we describe the five-year medical history of a Japanese woman and her father with RTH and coincidental chronic thyroiditis. The woman, aged 28 years, was referred to our hospital because of suspected hyperthyroidism. She showed a normal level of TSH and elevated levels of free triiodothyronine (FT3) and free thyroxine (FT4). Anti-Tg and anti-TPO antibodies were slightly positive. Since RTH was suspected, her parents were investigated with informed consent. Her father showed elevated levels of TSH, FT3 and FT4, and was positive for both anti-Tg antibody and anti-TPO antibody. Her mother had hypothyroidism caused by chronic thyroiditis. Sequencing of the TR beta gene showed that the patient and her father had a codon 453 mutation resulting in a CCT (proline) to ACT (threonine) substitution. The patient gradually developed emotional disturbance, and was admitted to a psychiatry ward for two months, where she was treated with lorazepam and her condition improved. Her father, on the other hand, has been doing well for five years. The patient and the father showed different clinical courses, even though they carried the same mutation of the TR beta gene. The fact that the father showed an elevated TSH level, whereas the patient did not, was thought to be due to decreased thyroid function caused by chronic thyroiditis.