Muscle crap can occur as a side effect of levothyroxine. This is temporary and shall resolve in few weeks once you reach a stable serum TSH. Other possible side effects of levothyroxine are nervousness, excitability, tremor, muscle weakness, fatigue, cramps, sweating, flushing, heat intolerance, headache, insomnia, tachycardia, palpitations, restlessness, depression, difficulty in sleeping, and excessive weight loss. Even if any side effects occur, this is seen in early stage of treatment and often resolves with progress of time once you achieve a stable dose of levothyroxine. So monitoring of treatment is essential. Serum TSH should be measured to monitor for adequate replacement. Advice 1. Continue levothyroxine as advised 2. Take rest for few days 3. Apply any over the counter muscle relaxant cream such Relaxyl ointment three times daily 4. Measure your serum TSH after 6 weeks; take the report to your doctor. Doctor will titrate the dose of levothyroxine as per serum TSH level 5. Regularly check your blood pressure and pulse rate 6. Avoid smoking, coffee and alcohol I hope this information will help you. I will be happy if you ask another question Thanks