thyroid levels in healthy people-do they vary?

I just imagine that thyroid levels in healthy people varied according to
-season
-time of day
-temperature
and maybe other things beside
Does anybody know or know where to look?

Is there any data to say that long term survival rates after thyroidectomy are equal to people without thyroidectomy, or would we expect to lose a year or two of life expectancy?

(You might guess that I'm being told it may be a thyroidectomy for me - Graves Orbitopathy/Ophthalmopathy apparently still active after >2years, evidence being doppler, TRAC and gritty eyes)

Thanks