Thyroid hormones fluctuate daily. weekly, monthly etc. If you're always in the normal range which the overwhelming majority of people are, thyroid hormones mean sweet FA for anyone. Sure BMR varies person to person... by about 100 cals. There's studies I've posted before finding no significant differences between people of similar height, weight etc. If you're 6'5 and hyperthyroid vs 4'10'' and hypothyroid then fair enough there's going to be a hell of a difference but as the majoirty of people are young, the reason they're fat is eating too much, the reason they're too thin is because the appetite is that of a shrew.
I've never seen any evidence showing much in the way of different T4/ 3 levels having any effect on metabolism; if you have seen a few studies, would be interested to read of course
If we were discussing middle age women, whole different topic. Hypothyroid common and menapause effect to boot. Males to a lesser extent.