Whatever they actually meant, I think they should have meant body fat, with what constitutes "excess" varying from person to person. Past a certain level of storage in fat tissue itself, excess fat starts to accumulate in all sorts of lean tissue, muscle, liver, heart, pancreas. With the exception of some increase fat in muscle in athletes, it's generally a bad deal and interferes with mitochondrial function etc.



I have seen suggestions that for people with heart failure, reduced energy due to hypothyroidism could make things worse. You could see a vicious cycle, heart failure reduces oxygenation of blood, higher fat vs. carbohydrate increases reliance on oxygen for energy supply. A ketogenic diet is different vs. a diet that's just high in both fat and carbohydrate, Dominic D'Agostino's work on the ketogenic diet started with the observation that people and animals were more resilient in low oxygen conditions when ketones were elevated.

Hypothyroid also seems to increase risk of atherosclerosis, and there seems to be some effect on vasodilation. I've only skimmed through this, but it's interesting;

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4318631/

"Thyroid and the Heart."

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4590606/



I haven't really looked into thryoid's effects on mitochondria. There's just so much of this stuff. This looks relevant, I haven't read it, just sticking this here to read later and in case anybody else is interested.