How many weeks after beginning 25mcg of Synthroid (thyroxine) can side effects occur?

Thyroid treatment. 25 mcg of levothyroxine is a very low dose; a normal dose for hypothyroidism is typically much higher. The only side effect is taking too much (hyperthyroidism: palpitations, sweating, tremor, etc) or not taking enough (hypothyroid: fatigue, cold, etc). But such symptoms can be from other things. You need to get levels checked and talk to the doctor who initially prescribed this.
Synthroid (thyroxine) If your doctor prescribed the best dose for your thyroid disease you are unlikely to get any side effects. If you are getting side effects let your doctor know.