After four useless GPs, the fifth one found I had an underactive thyroid. At the time of diagnosis I could barely get out of bed, couldn't eat as I had zero appetite, didn't even care that I was ill because I just wanted to sleep, hair fell out, anaemia, no eyebrows, hair falling out, felt like I'd done a full day in the gym the day before, could be in the middle of a sentence and didn't know what I was talking about, couldn't remember anything and slept about 18 hours a day (Had given up work at this point). I actually thought I was dying but felt totally detached and had no feeling about it either way. I even got to the point of wondering how I could be involved in a serious accident that only hurt me so I could lie asleep in a hospital bed and not feel guilty on my husband.
The wonderful fifth doctor put me straight on thyroxine and looked after me for many years. I felt a million times better but have never felt completely 'normal'.
My advise to you, get yourself completely educated, know your own personal blood levels (for TSH, FT3, FT4 etc.). FB has some really good groups with very knowledgeable people. My levels look too low on paper but that's when i feel at my best, i won't necessarily be allowed to keep at these levels depending on the GP. It affects all sorts, your ability to absorb iron, your eyes and loads more, have a good read.
Be prepared to go to your GP armed with credible evidence and don't be afraid to ask questions, question their judgement and fight your corner. Unfortunately GPs don't seem to know enough about hypothyroidism.
Like others, I don't eat much dairy, only cheese occasionally, this made a huge difference. Research how to take your thyroxine, at least an hour before or after food and most drinks except water so that it can be absorbed correctly in the gut and take with a full glass of water. I take mine just before bed, I found switching from taking it in a morning made such a huge difference to me, others doing this got insomnia, everyone is different. Face the fact that you are on medication for life, take your medication as directed, it is so important.
Good luck with your journey if this is hypothyroidism, at least if it is, the only way is up. I am 15 years down the line and wish I had advise at the beginning.