SPINA-GT estimates the maximum thyroxine your thyroid can secrete per second, calculated from two ordinary blood results — TSH and free T4 — drawn in the same sample, so it needs a blood draw and a lab report, not a scanner. Unlike either number alone, it describes the gland itself: a capacity, in picomoles per second, not a snapshot of hormone levels.
That matters because the pituitary compensates: a gland losing reserve is driven harder by TSH and, for a while, keeps free T4 in range. SPINA-GT reads the effort behind the result.
What you need
- TSH (thyrotropin) and free T4 (free thyroxine), from the same blood sample and the same laboratory.
- The units printed on the report: TSH in mU/L, free T4 in pmol/L outside the United States and in ng/dL inside it. Enter each in the unit the calculator asks for.
- A steady state: no change of thyroid medication in recent weeks, and no acute illness around the draw.
- A consistent draw time: TSH peaks overnight and drifts down through the day, so compare morning with morning.
- Anything that shifts the pair: biotin, amiodarone, lithium, glucocorticoids, dopamine, estrogen therapy, pregnancy.
How the measurement works
The thyroid and pituitary form a feedback loop, so the informative quantity is the pairing — how much free T4 your gland delivers for the stimulation it receives. Ample thyroxine on little TSH means reserve; the same thyroxine only under heavy TSH drive means the gland is near its ceiling.
SPINA-GT formalizes that with a model of the loop, adding assumed constants for binding proteins, distribution volume and hormone clearance, to return a secretion rate. Each input pushes one way: at a given TSH, a higher free T4 raises the calculated capacity; at a given free T4, a higher TSH lowers it. The Maximum thyroid secretory capacity calculator holds the published constants and unit conversions, so you supply the two lab values and it does the arithmetic.
How to read the result
The output is a rate in pmol/s. The reference interval usually quoted for SPINA-GT is roughly 1.4 to 8.7 pmol/s, from the original validation work; because it depends on the free T4 assay and the model’s assumed constants, treat it as the calculator’s frame, not a universal band. Read the pattern, not the decimal.
| Pattern | Usual reading |
|---|---|
| Capacity below the interval, TSH raised | Less thyroxine per unit of stimulation, the picture of established hypothyroidism |
| Capacity low but inside the interval, TSH and free T4 both in range | Reserve trending down before either routine number leaves its band; repeat before acting |
| Capacity above the interval | Output beyond what the loop is asking for, as in autonomous nodules or Graves’ disease |
A single value carries noise from two assays at once; a repeat in the same laboratory says more than one number against a published range.
What it does not tell you
The calculation assumes an intact feedback loop in a steady state. In pituitary or hypothalamic disease TSH is inappropriately low for a low free T4, so the capacity can look normal while the gland is under-stimulated. When TSH is strongly suppressed it sits in the model’s denominator and the value turns unstable. And on levothyroxine the model cannot separate tablet-derived thyroxine from what the gland itself makes, so it overstates the gland.
It inherits both assays’ flaws: biotin, heterophile antibodies, non-thyroidal illness, dopamine and steroids all move TSH or free T4, and binding proteins shifting in pregnancy or on estrogen break the model’s assumptions. Free T4 methods are not interchangeable, so capacities from different labs are not comparable.
Even with clean inputs, SPINA-GT covers one step: making thyroxine. It says nothing about how much T4 your tissues convert to the more active T3, which a Peripheral thyroid hormone conversion assessment addresses, or how strongly pituitary and tissues respond to the hormone they receive, the domain of Thyroid hormone sensitivity. It names no cause: autoimmunity, iodine deficiency, surgery and radioiodine all lower capacity, and only antibodies, imaging and history tell them apart. It is not a diagnosis, a treatment threshold or a measure of biological age, however closely thyroid hormones track metabolism. Guidelines still decide on TSH and free T4; this is a research lens over them.
Related reading
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.