Transdermal Warning
Unfortunately, one of the biggest drawbacks of transdermal HRT (especially sprays) is that it can contaminate blood tests.
When you apply a spray, it mists a wide area in extremely absorbable spray. This cannot be washed off and will remain for about 2 weeks. It's not strong enough to affect your levels but will get picked up when the skin is punctured by a blood-draw needle.
It may be possible for standard venous blood draws from the elbow to work if HRT is applied to the armpits, thighs, or scrotum. However, finger prick tests will definitely provide an inflated value for the hormone you're applying. Randox Health's QuickDraw may work.
This doesn't make blood tests useless. They are still an important tool for monitoring various HRT-related health risks, especially in transmasculine people.
Useful site: https://transharmreduction.org/blood-tests
What to test
Blood test markers in rough order of priority:
Transmasculine tests for efficacy
E2, T, DHT
Transmasculine tests for safety
FBC (primarily Hematocrit), LFT, HbA1c, Lipids
Transfeminine tests for efficacy
E2, T, DHT, Progesterone, PRL, SHBG, LH, FSH
Transfeminine tests for safety
PRL, E2, FBC, LFT, HbA1c, Lipids
If on sprays/gels:
Transdermal forms of HRT can contaminate results, making them appear much higher. Instead, SHBG can indicate estrogen level and LH/FSH can indicate effective testosterone suppression.
Where to get tested
First option: your GP. They may take convincing, so bring supporting evidence (like NHS GIC guidelines) and be prepared to go through multiple doctors. Most GPs are more likely to help if you've started HRT. Consider speaking to local trans people through trans networks or at pride events to find out which practices cooperate.
GP WARNING
Recent community reports suggest that minors who tell a GP they're considering taking DIY have started receiving safeguarding referrals. Consider this risk before speaking to your GP.
If you're nearby: CliniQ or 56 Dean St in London. They both provide free, NHS-standard blood tests and don't require any action from your GP.
Also possible: order a finger-prick blood test online from these sellers. This will result in hormones you apply as sprays showing up to an order of magnitiude higher than is in your body.
Blood test data is always welcome if you're comfortable sharing. It helps me calculate normal doses for the sprays and further understand the variation of absorption.