What becomes readable

Physical signals

no chemistry needed

The magnets ride in your blood, so every heartbeat sweeps a surge of them past the coil. These come free with the signal.

Heart rate Signal swells once per beat Free

Pulse waveform Shape of each beat, not just the rate Free

Heart rate variability Beat-to-beat timing Free

Core body temperature Magnetic response shifts with temperature Free

Blood pressure Via pulse timing between two points Plausible

Abundant

millimolar — millions of molecules per sensor

Enormous headroom. These are the easy wins and they'd work first.

Glucose 4–6 mM Yes

Lactate 0.5–2 mM at rest, up to 20 mM working Yes

Ketones 0.1–3 mM Yes

Alcohol mM while drinking Yes

Sodium & hydration 135–145 mM Yes

Urea (kidney) 2.5–7 mM Yes

Calcium 2.2–2.6 mM Yes

Magnesium 0.7–1 mM Yes

Uric acid 200–400 µM Yes

Creatinine (kidney) 60–110 µM Yes

Reachable

nanomolar — above the 1 nM detection floor

This is the interesting tier. Nothing on the market gives you any of these continuously.

Cortisol the prize 100–500 nM · the actual stress hormone, not inferred Yes

CRP — inflammation the surprise 9–26 nM normal · up to 900 nM when sick Yes

Vitamin D 50–125 nM Yes

Testosterone 10–35 nM Yes

Melatonin ~0.3 nM at night peak — right at the floor Edge

Out of reach

picomolar — too rare to clump a sensor

Fewer than one molecule per sensor. Nothing to grab onto. These need a further invention — see below.

Insulin 20–200 pM No

Estradiol 100–1500 pM No

Thyroid (TSH, free T4) 1–25 pM No

IL-6, TNF-α ~0.05–0.5 pM No

Ferritin 44–660 pM No

Sleep isn't one measurement

There's no accelerometer, so there's no movement data. But you get the four signals a watch uses to guess at sleep — measured instead of inferred.

Heart rate & HRV — from the pulse ripple, all night, nothing worn
Core temperature — drops at sleep onset, rises before waking. A real circadian clock.
Cortisol rhythm — should bottom near midnight and climb before you wake. This is the actual hormonal signature of a recovered night.
Melatonin — sits right at the detection floor. Might work, might not.

What would unlock the bottom tier

One trick: catalytic amplification — make a single analyte molecule trigger many sensors instead of one, the way one enzyme chews through thousands of substrates.

That's what moves insulin, thyroid and the cytokines from out of reach to abundant. Nobody is currently doing it in the magnetic domain. It's the single highest-value open problem here.