I have recently developed terrible fatigue. I never sleep during the day, but recently I find myself on the sofa feeling totally fatigued and nodding off. I haven’t increased my training volume, and if I compare to this time last year, I am lucky if I have a weekly TSS of 200 compared to 350 last March. I am arranging some blood tests, but I wondered if those who are very fit and active should look at different markers?
I’m 52
Go see a doctor, don’t ask the internet, don’t do random blood tests but have an actual doctor make a differential diagnosis and test that, especially when there are no other major life events/ stressors
It really is see a doctor and investigate.
Save yourself a step- See if the doc can order the blood work before you show up. Get the blood drawn at the lab, get results, then talk to doc.
I always find more useful to listen to people’s experiences. Because doctors will just ask for basic blood metrics and will say get more rest when they can not find anything on blood.
Can you think of any reason why you might be more fatigued? For example, has anything in your life changed, work, relationship, stress levels? Have you increased training or dtartes doing something new? Has your diet changed? Have you recently been ill?
If none of that, go see a doctor.
This could also be a factor to consider. I’m a few years older than you and I’m constantly tired, at least that is how it feels…
Things to keep an eye on are sleep quality, resting heart rate, stress in general etc.
Include tests for Epstein Barr and Roseola. Both are childhood viruses that can reactivate, especially when stress levels are wand immune function is low. Check for systemic inflammatory markers like Interleukins (especially IL-6) and cytokines. May also want to arrange a low level aerobic test to look at fat and carbohydrate metabolic function
At least they will have excluded most serious underlying issues
Where can you have this done? I live in the UK, and I am not sue tests like that exist on the NHS.
I’m far from 52 but I highly doubt you can feel great at 51, then within the next year age comes and kicks you so much that you do 1/3 less TSS and you feel completely wiped. Something else is definitely going on.
All of this is important but I’d probably still go see a doctor.
Maybe look into Lyme disease also. I’ve seen many people suddenly be super fatigued and turns out they have Lyme.
Could happen at about 52, but only if you’re female. Don’t know if the OP is male or female.
Try and get the GP to tick as many boxes as possible on the blood test sheet. If you say you might have been bitten by a tick, they should check for Lyme disease.
As someone said above, at least a blood test can rule out some of the more serious things. It can also show up something relatively simple, for example low iron levels.
Also even if it doesn’t show up anything, you still need to do it first, before you can ask for further tests.
Please please see a doctor 
How long have you been feeling this fatigue? And has something happened to potentially create it?
Sure, it’s not likely but I can assure that it can happen. In fact I was so fatigued from work-related stress at one time last year that I had to bail after about 20 minutes from a z2 workout last summer. A disastrous recruitment of a new team manager detailed the 2023 season for me. It took about 4-5 months to recover from that experience. But to be fair, if something similar happened to the OP they probably would’ve been aware of the root cause.
In the States, it would be through your primary care physician or a specialist. Not sure about the UK system. Maybe search for a physician who specializes in Functional Medicine
Before you do this make sure your basic blood panel like Hb, Ht, sedimentation, CRP, white blood cells, hb1ac, hormones, Na, Ca, K and Mg etc is in order (otherwise ask @HLaB for what can cause fatigue! but of course it could also be something as simple as a hypothyroidism)
It is difficult to know how much daily stress can impact your exercise capacity. What has not helped has been repeated viral infections since covid came along. Mainly, these have been chest infections, but the last one I had was in late January, and although it knocked me for six, I did get back training ok.