Question: Dr. Cwynar, I love your column. I take 50 mcg a day of Synthroid. I recently read this drug has a half-life of 7 days. I’m worried that I’m getting too much Synthroid. At what point does it become toxic?

Dr. Cwynar's answer:
Everybody's requirements are different for thyroid replacement. You are correct in your comment that Synthroid has a half life of 7 days. It takes three half lives -- three weeks -- for the hormone levels to plateau in the body. So you will want to take the Synthroid for at least three weeks before adjusting your dosage. After that point you should have a thyroid function test done. This is blood work to check for the thyroid hormones TSH, free t4, and free t3. You should get these labs every six months while taking Synthroid. Your dose of 50 mcg is very small but still should be monitored. Some patients actually need 400 mcg