Thyroid - How long does it take for thyroxine to work?

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I've been prescribed thyroxine for an underactive thyroid. I've been taking it for a week now and haven't noticed much. How long have people found it takes before things start to improve, please?

I've been taking thyroxine since 1993. it must have made a difference because dh remembers me going to bed at 6 pm on Fridays and sleeping till 1 pm the following day (this was before dc were born, obviously!) and I don't do that now, but can't remember when I started to have more energy. Think it was v gradual.