You could post your thyroid results for a second opinion as to whether you're optimally medicated?
Otherwise you can think about the following.
You can check your vit levels (folate, B12, Vit D, Iron/Ferritin). Being low in any of these will cause tiredness. Many of us us hypos are particularly prone to deficiencies and these are often missed or ignored by docs so post these results if you have them also.
You could also think about the times it is happening and see if anything relates - Blood sugars often take a dip at this time of day and they are easily tested at home at various points during the day with a diabetic finger prick test kit - it doesn't cost that much for a three day review kit. There are things you can do to help stabilise these levels if turn out to be the problem.
Do you eat enough calories to cover your bodies energy requirements.
Also does it happen more often after you eat or eat specific things. For instance, do you skip breakfast and only start to feel tired after lunch. If you eat anything that you are intolerant too, it will have this type of effect. I worked out that if I skipped lunch, it was less likely for this to happen to me - this was only an experiment for a day here or there - I certainly don't mean for you to start skipping meals lol!!
I also had a sleep disorder until I gave up gluten and was always tired but a little worse after eating and always falling asleep on my desk more at this time. I would sometimes wake up a little more around 10pm and after some time of eating any food.
If none of those pick up anything, then you could maybe check your cortisol levels throughout the day as these are always cyclical in nature too.
This is all I can think about at the mo but hope it helps