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What are the differences between T3 and T4 thyroid hormone?

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Dr. Albana Sejdini

General & Family Physician

Practicing since :2006

Answered : 6025 Questions

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Posted on Tue, 20 Oct 2015 in Thyroid Problem and Hormonal Problems
Question: I am currently taking 50 mcg dose of Synthroid, but surfing the web/net trying to find out what's the difference between T4 and T3 and wondering which might be better for me? There's so MUCH information on the internet I'm getting lost.
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Answered by Dr. Albana Sejdini 2 hours later
Brief Answer:
Synthroid is the synthetic thyroid hormone replacing T4...

Detailed Answer:
Hi xxxxx,

Thank you for your query and for asking about meds you take and their effect on your body.

Synthroid is the brand name for the generic Levothyroxine, which is the synthetic hormone made my men and used to replace the function of normal thyroid hormone T4, which might be produced in lower titer or not produced at all for many reasons.

Both T4 and T3 are thyroid hormones produced by thyroid gland. T4 is produced in larger amounts than T3, because T3 :
- has much more strong activities than T4
- is produced more by conversion of T4 in the peripheral cells (outside the thyroid gland).

So, usually, in a replacement therapy, we use the main thyroid hormone levothyroxine (T4) as it will be converted into T3 in the periphery.

Try not to get stressed or overlook at the info you see in the internet. Focus more on patient's guide as it will help managing your current health problems.

Hope it answered to your queries! Let me know if you need further assistance!
Dr.Albana
Above answer was peer-reviewed by : Dr. Chakravarthy Mazumdar
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