I'll mention that you don't need to add any more stress to yourself then, right?

The surgery was a stress on your body.
The partial thyroid the body must use is a stress.
Stop smoking is a mental stress, body has withdrawal too though, so some stress there, but offset by the benefits.
Diet is a stress too.

Which of those can you control?

Diet, probably sleep too. But for sure diet.

What would be a reasonable deficit for others with an otherwise healthy body - doesn't apply to you now.

Plus, how much were you eating below maintenance on average to lose that weight?
And did that daily burn include the exercise that MFP did NOT consider when making the diet goal?

Merely undereating too much can affect the thyroid, and mental state too.

When was your last diet break since October?

I think you should be very proud of your accomplishments so far with your weight loss and stopping smoking. Like heybales said, the stress can stop your progress right in it's tracks and you have a lot of stress going on right now. Try to alleviate the stress where you can and get plenty of rest while you heal from your surgery. Having too large of a calorie deficit is a huge stress too. Can you give a little more info on your stats (age, height, weight, activity level) and how many calories have you been eating? We can help you more if we know more information about you.