All the above is correct, but sometimes nailing down the origin or excess RT3 can be easy, sometimes it can be complicated. Having the flu, being sick, having an infection, or healing from a major injury can all contribute to increased RT3. Then again, so can iron/ferritin, adrenal issues, metabolic disorders and electrolyte imbalances.
The labs you provided obviously present that you have higher than desired RT3, in fact your FT3/RT3 ratio is 10.8, which should be > 20. Knowing what you know now, there could be some deeper discussion(s) with more labs, i.e., iron/ferritin, metabolic, CBC's, adrenal profile (4x saliva), stats, current conditions & family history that might be relevant ...? Just more stuff that might help you get a handle on this.