TDEE significant drop?

Yes, eating very low over a long period of time can lower your BMR and your TDEE. Weight also affects BMR and TDEE. Unless you actually had yours tested, the reason why you are seeing this difference is simply your change in bodyweight.

Your BMR and TDEE are lower because you've lost weight...ergo, your body doesn't require the same amount of energy that it needed when you were heavier. The leaner you are, the less calories you're allowed.

Sh!tty but true

Your TDEE definitely dropped because you chose to go on a very low calorie diet, and you lost a ridiculous amount of weight in a short period of time. While that may look good on paper, such quick weight loss means that a significant portion of your weight loss was muscle, which is not optimal, because muscle mass drives your Basal Metabolic Rate, and since that dropped, your TDEE has dropped.

Your TDEE definitely dropped because you chose to go on a very low calorie diet, and you lost a ridiculous amount of weight in a short period of time. While that may look good on paper, such quick weight loss means that a significant portion of your weight loss was muscle, which is not optimal, because muscle mass drives your Basal Metabolic Rate, and since that dropped, your TDEE has dropped.

Strong broscience

Thanks for the info. To be honest, I got really scared when I read your reply about how most of my loss was muscle mass. I can't see big differences in my posture and build except that I look thinner and my cloth sizes went down significantly but your explanation about my constant loss makes sense.
I started at around 33% body fat (obese) and I think I'm around 23-25% now which is still considered overweight.
Could it be that because of my obesity, most of the 43 pounds were fat and the muscle loss was minimal?

Oh, never most, either by pounds or %. But more than you would have liked and more than was needed.

At the start, yes, when you really had a deficit where the math works out for the 3500 cal / lb of fat for energy. But even some then was going to be muscle unless progressively lifting heavy and eating enough protein.

Though even in that case, you don't lose what you use, but your body can start deciding what is used most compared to least.
Muscle breakdown happens no matter what, all the time, diet or not. The amount of diet and exercise and protein just help determine what the body can rebuild again, either all or almost-all.

But as you lost more fat and weight, the amount of almost-all being re-built again would become less and less. So there is additional weight lost, but smaller % from fat.

The type and amount of exercise and protein would really determine how bad an effect.

Lots of intense cardio and low carb diet - goodbye muscle easily.
Lifting only reasonable amount of time and high protein diet - not so bad at all, depending on other stress your body is dealing with.

I apologize for plugging that "most" in my reply. You never said that.
I have been lifting + doing elevated pushups at least 3 times per week and my protein intake has very rarely gone below 60g per day (on the ~1300 calories diet).
You seem to be very well informed on this subject and I would like to get an idea of how much muscle mass I have possibly lost,
Possibly 10 pounds out of the 43 pounds? More? Less?

Again, thanks for the info everyone. I'll work on increasing my intake (specifically protein) and lift. :flowerforyou:

60g may be the RDA minimum for normal diet, but not enough when in a deficit.

Easy method is 0.82 g per lb of weight, or about 1 g per lb of LBM if you have estimate of bodyfat%. Change the goals for macros to whatever % makes those grams happen. Or the spreadsheet spells it out too.

Really wouldn't be able to know for yourself, only it's a whole lot harder building it back up.

Here's study of guys, starting lifting, eating at maintenance, that increased LBM (not just muscle which is merely one part of LBM, they didn't measure for muscle mass specifically sadly, so it would be less than this) at the rate of 3.5 lbs in 16 weeks. Not even a lb a month of LBM, less for muscle mass though some.
http://www.myfitnesspal.com/topics/show/778012-potential-muscle-gain-lifting-and-metabolism-improvement

For myself, trying to pay attention but getting into training for a triathlon too much, lost 2 lbs in 6 weeks of LBM. Considering I was topped off with glucose stores prior to weigh-in day, that was not a loss of just glucose, but muscle and whatever glucose it would have held. So mainly muscle. Lost about 6 lb fat though, so that's good.
And post discovery, I could go back to my records and the 3 bad intense cardio days in a row, and see exactly why it happened. A little week after week. Only 200 cal burned of muscle each week, probably 100 cal worth each day. Adds up.

I was underestimating my calorie burn based on old tests, and so when I thought I was eating enough carbs before the next workout, I was very short. Which means I touched on muscle destroying mode 1-2 days each week easy. So I accomplished what can be done with merely diet.