Carol's points above eerily gel with what I am going through at present! I've not got to the stage yet where my boss is getting to the handing out warning stage but I know my performance is well sub-par compared to normal as I seem to be in a permanent state of confusion ... 'brain fog' is exactly the term for it :nods:. I've been putting it down to the blood pressure meds I'm on but maybe a vitamin deficiency is also putting it's iron in the fire?
Definitely get blood work done, for a few reasons. A symptom is not the same as a diagnosis. If it is Vit D or thyroid (or some other things), you have a better chance of learning what the root cause is and how to address it. If the symptoms are actually from something else...e.g. Rx med reaction, you can explore that as well.
If it is Vitamin D...I don't think treating a clinical deficiency is a DIY process, for a few reasons.
Even though D is liposoluble, it takes a long time for the body to build up levels. Good to get a doc's opinion on what dose/frequency is best for your needs.
Some people who bottom out with a severe deficiency may have to resupplement for longer than the usual 3 months. IF this is the case, you need to know that, and so does your doc.
I don't know about UK, but here you can find up to 10,000 IU at a health food store. I have tried the higher strength OTC vitamins in hopes of saving myself a return trip to the doc and was very unhappy with the results. I had some very strange reactions to some OTC supplements, esp. at higher doses. Some caused a minor spike in my blood pressure, others upset my stomach, etc....issues I don't have on a (usually higher) strength of the Rx vitamin. I don't think this is the vitamin itself causing the reactions, but the binders and impurities used in the manufacturing process. OTC vitamins are unregulated here, there can be much more variables in what you get inside the bottle. Other folks may have different or better experiences, but I don't plan on repeating mine if I can help it
Good luck and hope you feel better soon!