You’re very fortunate to have been able to remain lean while eating as you liked all this time, relying only on training. This comes with being genetically gifted rather than being the norm.
It’s likely that at age 40, this now will not be entirely the case and to get back to being lean it will be necessary to avoid increases in calories while seeking to lose fat.
Some fat loss supplements are somewhat effective in reducing appetite, as well as raising metabolic rate, or in some cases, even improving metabolic processes. Where caloric intake remains the same, the effect of nutritional supplements on fat loss is usually less than that of properly dosed thyroid hormone. However, where the supplement product has significant appetite suppressant properties, and in the individual case the person actually eats less, this can make a big difference to fat loss.
What likely happened for you with Cytomel T3 Liothyronine sodium was that food intake went up to match increased rate of calorie burning, so you had no fat loss.
With T4, the dose was likely so extreme that you had fat loss anyway.
It can’t be stressed enough how potent thyroid hormones are. They absolutely must not be overdosed. Powders should be never be used unless a person has the means to dose extremely small amounts with 100% certainty.
I recommend not exceeding 50 mcg/day with T3, divided preferably into at least 3 doses but certainly at least 2 doses. With T4, I recommend not exceeding 200 mcg/day.
T4 tetraiodothyronine Levothyroxine is longer-acting than T3 and therefore is more convenient to use. It requires conversion in the body to T3, which for some individuals can be a limiting factor. For them, Cytomel can be preferable. It seems that is not an issue for you, so you could use T4.
But next time, keep the dose limited to the above, and make sure not to allow caloric intake to increase along with your increased metabolic rate.