Brief Answer:
Neuropsychological tests for dementia

Detailed Answer:
I read your question carefully and I understand your concern. Given the diagnosis of dementia in a close family member and at an early age, your worry is understandable.

It is true that there are no routine laboratory tests for dementia. However there are neuropsychological tests which do assess your cognitive functions, sort of like questionnaires and tasks to assess different higher mental functions. There are many available, the most common screening test is the MMSE (mini mental state exam). It is easy to administer, your general physician can do it himself. If it is outside the norm there are more in depth tests, best done by a neurologist or neuropsychologist. So that is the first step in investigating dementia. If those reveal abnormalities other tests may be ordered such as imaging tests which while not making the diagnosis support it.

That being said about dementia, I would add though that looking at your symptoms, other conditions and medications there are several which can justify your symptoms just as well and should be taken into consideration.
First low thyroid function is a possible cause, it is a known cause of dementia as well. Perhaps your internist has that already covered but thought I'd still mention it.
Another possible culprit may be the too many medication. You take not one but 3 drugs which act on the central nervous system, bupropion, gabapentin and duloxetine. They all can have side effects due to their effects on the central nervous system, effects such as fatigue, somnolence etc. So it can be discussed with your doctor the possibility of reducing or removing one of them.
Lastly depression itself can impair concentration and thus create the false impression there is a memory impairment.

I remain at your disposal for other questions.

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Brief Answer:
Read below

Detailed Answer:
Thank you for the additional information.

I imagined you took levothyroxine, it substitutes the thyroid hormone. The thing with the dosage is that there is not a fixed dose for all individuals, so it should be monitored as a higher than needed dose may cause high thyroid function, and a lower one causes a low thyroid function. It usually is monitored by checking the levels of TSH, a hormone which levels are dependent on thyroid hormone. That is what I meant that your internist has probably got that covered, meant that he has checked the TSH and adjusted the levothyroxine dose accordingly.

As for Lamotrigine, that adds to the mix a 4th drug which acts on the central nervous system and can affect cognitive functions, cause somnolence, fatigue etc.

So my initial hypothesis remain the same ones, but the addition of Lamotrigine makes the medication related hypothesis even more possible and you should really discuss with your doctor whether a reduction is possible.

I hope to have been of help.

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