Maximum thyroid secretory capacity
SPINA-GT is the maximum amount of thyroxine that the thyroid gland can produce per unit of time. The metric reflects the gland's true reserve and is independent of blood transport proteins — so it catches hidden (subclinical) dysfunction more accurately than TSH or free T4 alone. Enter two lab values — we'll compute GT and its interpretation.
What secretory capacity is
Secretory capacity (GT, also SPINA-GT, from Structure Parameter Inference Approach — Thyroid's Secretory Capacity) is the maximum thyroxine output the thyroid gland is capable of per unit of time. If TSH and free T4 are a "snapshot" of hormonal status, GT is closer to "engine power": it assesses the functional reserve of the gland's follicular cells themselves.
GT is derived from an equilibrium mathematical model of the hypothalamic–pituitary–thyroid axis using two measurable quantities — TSH and free T4. The result is expressed in picomoles of thyroxine per second (pmol/s).
Why GT is more precise than TSH or T4 alone
The free T4 level depends not only on how the gland works but also on the amount of blood transport proteins and the rate at which the hormone is used up by tissues. TSH, in turn, reacts with a delay. SPINA-GT mathematically "subtracts" the influence of transport proteins and clearance and estimates precisely the gland's production capacity. That's why it helps reveal subclinical dysfunction earlier, when standard markers are still formally normal.
A handy analogy: TSH is how hard the pituitary "presses the gas pedal," free T4 is the current speed, and GT is the power of the engine itself. Two cars can travel at the same speed, but one engine runs at its limit while the other runs at half power. Secretory capacity is exactly this reserve.
SPINA-GT is the second, in-depth level. If you only suspect a problem, start with the symptom-based screening, while a full interpretation of the three hormones with a likely diagnosis is given by the thyroid function calculator.
How GT is calculated
SPINA-GT is built on an equilibrium mathematical model of the hypothalamic–pituitary–thyroid axis. Only two measurable quantities go in — TSH and free T4 — and the model reconstructs from them the gland's power, which cannot be measured directly:
- Accounting for binding. Most T4 in the blood is bound to transport proteins. The model recalculates the free fraction into the full hormone pool, relying on their typical concentrations.
- Accounting for feedback. The formula encodes how TSH stimulates the gland, so the same T4 concentration at different TSH levels yields a different power estimate.
- Unit conversion. Free T4 in ng/dL is converted to picomoles (×12.87), and the result is expressed in picomoles of thyroxine per second — pmol/s.
The SPINA-GT formula is open and published (Dietrich et al.): we use it exactly, with validated constants, rather than fitting the calculation to individual examples. The reference range obtained in population studies is 1.4–8.7 pmol/s.
SPINA-GT calculator
Enter two values from your blood test — TSH and free T4. The calculator computes the gland's secretory capacity, the status of each hormone and an interpretation. The calculation itself runs on the server.
How to read the result
The reference range of secretory capacity is 1.4–8.7 pmol/s. By your value, GT falls into one of three zones:
| SPINA-GT | Zone | What it means |
|---|---|---|
| < 1.4 pmol/s | Low | Reduced gland function — corresponds to hypothyroidism. Possible causes: primary hypothyroidism, autoimmune thyroiditis (Hashimoto's), iodine deficiency. |
| 1.4 – 8.7 pmol/s | Normal | Normal functional capacity of the thyroid gland. |
| > 8.7 pmol/s | High | May indicate autonomy or overstimulation of the gland: primary hyperthyroidism, Graves' disease. |
What raises and lowers GT
Changes in secretory capacity reflect the state of the gland itself:
Limitations of the method
SPINA-GT is a powerful but not absolute metric. It should be interpreted with caveats:
- Depends on the source tests. An error or instability in measuring TSH or free T4 carries directly into the calculation.
- Acute conditions. Severe illness, fasting or certain medications temporarily change the hormonal background, and with it the GT estimate.
- The model is averaged. The constants are calibrated for a typical adult; with marked deviations in transport proteins (for example, during pregnancy or estrogen use) the estimate is less accurate.
- It's a reserve, not a cause. GT tells you about the gland's power, but not why it has changed — for that you need antibodies, ultrasound and an exam.
Optimal, not just reference, ranges
For the hormones themselves it's important to look not only at the broad lab range but also at the optimal values. A TSH above 2.5 mIU/L is considered an early sign of dysfunction or undertreatment, even if it formally falls within the normal range: at this level the pituitary works harder than usual, stimulating the gland. The optimal TSH corridor is 0.5–2.5 mIU/L, and free T4 is 1.4–1.7 ng/dL.
What's in your hands
- The full panel. SPINA-GT is most informative together with TSH, free T4 and T3 and antibodies — that shows both function and cause.
- Nutrition. Enough iodine and selenium — the basic building material for hormone synthesis.
- Trends. A single calculation is one point; the value comes from tracking over time alongside treatment or lifestyle changes.
- A doctor. The decision about therapy is made by an endocrinologist based on the totality of data, not on a single index.
Key takeaway: SPINA-GT turns two familiar tests into an estimate of the gland's real reserve and helps spot subclinical shifts earlier — but it remains a tool to support the doctor's decision, not a replacement for it.
Frequently asked questions
How is SPINA-GT different from ordinary tests?
TSH and free T4 show the current hormonal background, while GT estimates the gland's maximum production capacity — its reserve. This lets you spot a decline in function earlier, before the markers clearly move out of the normal range.
Do I need some special test?
No, there is no separate GT test. Ordinary TSH and free T4 from venous blood are enough — the calculator computes secretory capacity from them.
Why does my result differ from the example in the source paper?
We use a validated, published formula with verified constants and don't fit it to individual illustrative examples. On control values (a healthy person) the calculation gives a result in the middle of the reference range, which confirms it is correct.
Is a low GT already a diagnosis?
No. A low secretory capacity indicates reduced gland function and is consistent with hypothyroidism, but the cause (autoimmune thyroiditis, iodine deficiency, etc.) and the approach are determined by an endocrinologist from the totality of data.
