Levothyroxine is the standard treatment for hypothyroidism. It's a synthetic version of T4, the main hormone the thyroid normally makes, taken as a once-daily tablet — usually on an empty stomach.
The dose is adjusted using TSH blood tests until levels are right, and most people take it long term. It treats the hormone deficiency of Hashimoto's but does not cure the underlying condition. Never start, stop, or adjust it without a clinician.