A 52-year-old client reports weight gain and tiredness. On assessment, her vital signs are blood pressure 124/74 mm/hg, pulse rate 52 beats/min. respiratory rate 20 breaths/minute, and temperature 98.0 F. Laboratory results show low thyroxine (T4) and triiodothyronine (T3) levels. The nurse knows these symptoms are associated with which condition?