Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency care for severe trouble breathing, fainting, chest pain, severe confusion, extreme drowsiness, or rapidly worsening neck swelling that affects the airway.
Seek prompt medical advice for a new neck lump, rapid thyroid enlargement, worsening hoarseness, increasing trouble swallowing, or a persistent fast or irregular heartbeat.
Routine follow-up is appropriate for gradual changes such as returning fatigue, constipation, cold intolerance, menstrual changes, or mild neck fullness.
Arrange an earlier review if symptoms appear after a medication change or if doses have been missed regularly.
And for anyone, someone who:
The article also says
After levothyroxine is started or changed, TSH is commonly rechecked after enough time has passed for hormone levels to stabilize, often about six to eight weeks.
Once results and symptoms are stable, testing is usually less frequent.
The exact schedule depends on age, pregnancy, other health conditions, and whether medicines or body weight have changed.
People with thyroid antibodies but normal thyroid function may need periodic TSH testing, often around once a year when ongoing risk is present.
A clinician may recommend a different interval based on previous results.
Every sign above is quoted from the sofidex article on Chronic thyroiditis. If you are not sure, err upward — it is always reasonable to be checked.