Growth hormone.

It's very important to various parts of our body.

Including the thyroid: if you don't secrete growth hormone, your T4 (thyroxine, the almost inactive thyroid hormone) isn't converted to T3 (tri-iodo-thyronine, the much more active thyroid hormone). Which means that you're now hypothyroidal, even though all your thyroid tests come back as normal. That's because docs don't do the more expensive T3 test, they do the T4 test.

Nevermind all that: thyroid hormone levels are unimportant; what's important is that you do something about the problem.

Which is too little growth hormone.

Small kids make lots of growth hormone, all the time, but as they grow older they make less and less of it. Babies make enormous amounts, which is why some moms feel absolutely fabulous - as long as they're preggers. This source of growth hormone stops (for them ...) the minute they've popped, and now they crash. Badly. Unless, of course, they've kept up their own production of growth hormone. Smaller amounts, but still.

Adults start to secrete growth hormone about 30-60 minutes from the start of major or minor exercise, and they continue to secrete growth hormone for about an hour or so after starting said exercise. It doesn't matter, growth hormone-wise, if you're doing a 100 m spurt or if you're doing a 10 km walk; the amounts secreted are almost the same.
Adults also secrete growth hormone in the first phases of deep sleep.

In addition to keeping the dreaded hypothyroidism at bay, growth hormone is great for getting rid of general flabbiness. A 5-minute stint on the treadmill will help there, but I'd argue that a brisk half-hour walk outdoors, while the sun is out and about, is better every time. It's not always doable, but still, it's better.

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And it's interesting, isn't it, how too much insulin in your blood stops growth hormone secretions, and how too little deep sleep does the same, and how exercise without the pick-me-up sugary drinks afterwards really gets the production of growth hormone going ... (and if you now say: "Oh, but I never do sugary drinks, all of mine are diet stuff!" I suggest you read my Sweeteners vs. sugar rant.)

If you exercise and do sugars or simple carbs (during or after your exercise), you make no growth hormone to speak of. Ditto if you eat a load of simple carbs too close to bedtime. If you generally have too much insulin in your blood - no growth hormone. That last one? That's Syndrome X ... which, due to the idea that "fat is bad for you" (it's not), and due to a generally sedentary lifestyle, is all too common in the western world.

So ditch the fast carbs, start exercising and get real sleep, do vitamins and minerals, and see what's left of your tiredness, flabbiness, lethargy, cold-bodiedness and so on after a few weeks of that.

And do hang on in there, it's well worth it.