Hyperthyroidism means an overactive thyroid: the thyroid makes too much hormone, so the body speeds up. Typical signs are unexplained weight loss, a fast or irregular heartbeat, feeling hot and sweaty, anxiety, tremor, and trouble sleeping.
The most common cause is the autoimmune Graves' disease. It's diagnosed with a TSH blood test (TSH is usually low) and managed with anti-thyroid medication, radioactive iodine, or surgery. Some symptoms overlap with menopause.