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Biomarkers

SPINA-GT — the thyroid’s secretory capacity

The gland's maximum thyroxine output

The thyroid’s maximum capacity to produce thyroxine

SPINA-GT (GT, secretory capacity) is a calculated structural parameter reflecting the maximum stimulated thyroxine output of the thyroid gland per unit of time. In essence it is the gland’s “capacity” rather than the momentary hormone level.

It is computed from equilibrium TSH and free T4 using a homeostatic model (the SPINA method).

Reference ranges

Reference values and how it is calculated:

Reference1.41 – 8.67 pmol/s
What it reflectsmaximum T4 output per second
Inputsequilibrium TSH and free T4
Important: The calculation is done by SPINA Thyr software or an online calculator; the values are valid only when thyroid function is stable (at equilibrium).

What an elevated level means

A high GT indicates increased secretory capacity of the gland and is typical of hyperthyroidism — for example in Graves’ disease or a toxic adenoma. It reflects that the gland is able to produce excess hormone.

What a low level means

A low GT points to reduced gland reserve and is seen in primary hypothyroidism and in autoimmune thyroiditis with tissue destruction. The parameter may fall before changes in ordinary tests become obvious.

How the test is done

You cannot “order GT” on its own: TSH and free T4 measured at the same time are needed. The value is computed with the SPINA formula; peripheral hormone conversion is described by the related index SPINA-GD.

What affects the value

  • Taking thyroxine distorts the calculation (the gland’s own output is required).
  • Unstable function (recent treatment, acute illness) breaks the equilibrium assumption.
  • Values depend on the method and units used to measure free T4.

Calculate using this marker

Calculators where SPINA-GT (secretory capacity) is used directly:

SPINA-GT (Thyroid) The thyroid's secretory reserve, calculated with the SPINA model Open Comprehensive Thyroid Assessment Hormones, antibodies, and indices — TSHI, SPINA-GT/GD, and free T3/T4 Open

Frequently asked questions

How is SPINA-GT better than plain free T4?

Free T4 is the current hormone level, whereas GT estimates the gland’s maximum reserve adjusted for TSH. This helps reveal a fall or rise in function earlier and more precisely in borderline cases.

What does a low GT mean?

A reduced ability of the gland to produce thyroxine — as in hypothyroidism or autoimmune thyroiditis. The value should be interpreted together with TSH, free T4 and the clinical picture.

Related terms

TSH (Thyroid-stimulating hormone) The pituitary hormone that controls the thyroid gland Free thyroxine (T4) The main thyroid hormone SPINA-GD (deiodinase activity) Peripheral conversion of T4 → T3 TSHI (Thyroid Sensitivity Index) The adequacy of TSH to the free-T4 level Thyroid gland The endocrine organ that governs metabolism Hypothyroidism Reduced thyroid function with insufficient production of thyroid hormones, leading to a slowdown of all metabolic processes.

Sources

  1. Thyroid’s secretory capacity — Wikipedia
  2. SPINA.GT: Calculated Thyroid’s Secretory Capacity — SPINA R package (CRAN)
info
This content is educational and does not replace consultation with a physician. Reference ranges may vary depending on the laboratory and method — rely on the ranges stated on your own test report.
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