I have always had low thyroid, low tsh low t4, low t3 etc. I went through the whole routine of trying just t4 as first doctor wanted, t3 was still always low. Finally got a doctor to give me Armour and it was a game changer. That being said there was a problem with Armour for awhile years ago,improperly dosed, so he switched me to a synthetic t3/t4 and just stuck with it for years. I eventually just started using T3 and noticed no difference. As much as I needed to take of t4/t3 combo doctor couldnt get a balance to just supplement my natural production. So basically my thyroid is useless. So Ive took just t3 for years, I keep track and just try to keep my t3 in the extreme upper range unless its summer cutting season. For me around 50 mcg keeps me in upper range, i split the dose morning /night. When summer cutting maybe 100 to 125 mcg. Much more than that and my heart will race a little especially at night. I know some people say they feel better taking t3/t4 combo but i personally don't notice any difference. That being said I'm a slave to t3 like test for the rest of my life, the test problem I created, the low thyroid was just genetic.