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Thyroid Diseases

Synthroid and T3 and T4 and TSH



I understand that T3 is available through Synthroid. Shouldn`t T3 be tested along with T4 to make sure that one is not over converting? Is it true that high amounts of T3 can cause insomnia? What is the ideal level for TSH with hormone supplementation? What is the ideal free T-4 and free T-3 level? Thank you!


To answer your first question, any circulatory T4 will be converted to T3 but at a steady state so usually T3 level can be estimated based on T4 level except a few exceptions. As for the rest of your question, primary hypothyroidism diagnosed based on abnormal thyroid function test and specific clinical finding is treated with thyroid hormone replacement. The goal of this therapy is to create a normal thyroid function state that can be assessed by TSH level; generally serum TSH values between 1 and 3 mu/L are considered normal (95 percent of euthyroid volunteers have serum values between 0.4 and 2.5 mU/L). Unless, taking T4 is inconsistent, T4 or T3 are normal in view of normal TSH.

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Response by:

Marzieh  Salehi, MD Marzieh Salehi, MD
Assistant Professor of Endocrinology
College of Medicine
University of Cincinnati