Taking two thyroid pills in one day by mistake can result in an overdose of thyroid medication, which can cause symptoms such as rapid heartbeat, fatigue, anxiety, headache, sweating, nervousness, chest pain, and joint pain. These symptoms may not develop immediately and may take several days to appear. If you accidentally take a double dose of thyroid medication, it is unlikely to harm you, but you should contact your healthcare provider for advice on how to proceed. They may recommend skipping a dose or reducing your dose for a short period of time to avoid further complications. It is important to seek medical attention if you experience severe symptoms or if you have taken a large amount of thyroid medication.