@singingbones
Congrats on the great BMD improvements! How did I manage to forget this important part last time?!

My thoughts are pretty similar to yours - a blood estradiol level tells us how much of the drug is being delivered and absorbed from the patch - it does not directly tell us how our bone is responding to the estrogen, whereas BTMs reflect the state of bone remodeling in response to HRT around the time of the blood draw.

Also, from what I understand, for fracture prevention, neither a blood E2 level nor BTM is a validated target to adjust HRT dosing. The evidence that HRT reduces fractures comes mainly from the WHI, which used a fixed standard dose - conjugated equine estrogens (Premarin) 0.625 mg, rather than adjusting the dose to hit a particular blood estradiol level or CTX value. A 0.05mg/day estradiol patch is approximately equivalent to Premarin 0.625 mg. (For bone protection specifically, even lower patch doses are FDA-approved.)

So either approach (blood level or BTM) is being used empirically as a surrogate marker. The most established way to confirm that treatment is actually protecting our bones remains serial DXA scans

With all that said, a better-suppressed bone remodeling is associated with a high probability of BMD stabilization in clinical studies:
for transdermal estradiol patch - https://pubmed.ncbi.nlm.nih.gov/10831925/
for oral estrogen +/- progestin - https://onlinelibrary.wiley.com/doi/10.1359/jbmr.1999.14.9.1583

If you’ve been wondering, my 25 mcg patches put my blood E2 at 42pg/ml (trough, just one lab so far), CTX stayed consistently under 120 in year two. Now just waiting on judgment day… the DXA scan.