My DD9 was diagnosed with Graves (hyper) at 4 but has turned to hypothyroidism...(Hashimoto)...she takes her meds and seems to be fine. I have wondered if the hyperthyroid part ever played a part in her going through milestones quickly...as it makes everything in your body go faster...maybe it does your brain too?
I duno. I was hyper, too, because when you first get Hashimoto's you go through cycles of hyper / hypo as your thyroid is being destroyed. Hyper doesn't feel very good, either, and I think they both interfere with the ability to think clearly and work well.
I don't have hypothyroidism, but I have other medical issues that cause brain fog, and yes, it's awful. I really feel for you.
Do you do anything special to try to overcome it? I was having a series of really clear days for a bit so I worked on my business those days. Before I was diagnosed I was singing somewhat professionally. I started to think I really needed therapy because I felt so unmotivated and I could never count on my ability to sing the notes perfectly. I thought I was just crazy, but there was actually a hormonal explanation (well, mostly.

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I feel like this has really disrupted my attempts to achieve my goals.