Do you have some recent blood tests? I had a similar issue with t3, eventually my body got used to it. My heart would go 80, 90, then 110! I realized at some point I was checking my heart rate so often I was contributing to the high rate with my anxiety. Nonetheless it only lasted about a week then went down.
Do you have Hashimoto's? It's possible you're going through a flare up.
It's also possible you're overmedicated. It's so hard to say without some blood tests.
80-90 isn't incredibly dangerous. Do you have a doctor helping you through this? When I had my episode my Endo set up an EKG for me (ECG in English?) and determined it wasn't anything to worry about.
Is your heart consistently staying at a high rate? Or is it up and down? Is it correlated to your dosing time? (Happening about 1-2 hours after dosing and lasting a couple hours)
If it's maximum 90 there shouldn't be an immediate risk unless you have a heart condition. What was your normal resting heart rate before t3?
Good idea is to take your heart rate when you wake up, then an hour after t3, then a couple hours later. Then again later in the evening as the t3 leaves the blood stream. You'll get a good indication of how the hormone is affecting your heart.
(Sorry for the million questions)