Actually JPL is very correct with his statement, I have done much research on this topic and have used t3 doses at 150mcg for 4-6 weeks on end, and have done many cycles and always got blood work, not only have I discuessed this issue with 2 doctors I have many articles I will try to find, one is in the New Anabolic Reference Manual and every article stated how practically impossible it is to burn out your thyroid, my doctor said he has had pateints on thyroid meds for 10 years or more and once he takes them off their thyroid function is back to 100% normal after very minimal time. The body is very smart and in all of my reading I have yet to see proof of a thyroid being burnt out ( healthy thyroid) from supplementing with t3,t4 for longer periods of time, 8-12 + weeks. Like I said, I have done countless cycles at very high doses with blood work before and durning and after with NO problems at all.