SPINA-GD — the sum activity of peripheral deiodinases

Peripheral conversion of T4 → T3

The conversion of thyroxine T4 into active triiodothyronine T3 by deiodinase enzymes

SPINA-GD (GD, deiodinase activity) is a calculated parameter reflecting the maximum ability of peripheral tissues to convert thyroxine (T4) into active triiodothyronine (T3). It is an estimate of peripheral hormone conversion, not of the work of the gland itself.

The conversion is carried out by deiodinase enzymes (mainly type 1). GD is computed from free T4 and free (or total) T3.

Reference ranges

Reference values and how it is calculated:

Reference	20 – 40 nmol/s
What it reflects	maximum T3 output from T4
Inputs	free T4 and free (or total) T3

Important: The value is computed by SPINA Thyr software or a calculator; the result is strongly affected by acute and chronic illness.

What an elevated level means

A high GD means active conversion of T4 into T3. Elevated values have been described in obesity and metabolic syndrome, and in the early phase of Graves’ disease, when the demand for active hormone rises.

What a low level means

A low GD is typical of “low-T3 syndrome” (non-thyroidal illness syndrome) in severe acute and chronic disease, and in the first period after starting treatment for hypothyroidism. Persistently low values are linked to a worse prognosis in severe conditions.

How the test is done

There is no separate test: GD is computed from free T4 and T3 measured at the same time. The secretory capacity of the gland itself is assessed by the related index SPINA-GT, and pituitary sensitivity by TSHI.

What affects the value

Severe acute and chronic illness lowers conversion (non-thyroidal illness syndrome).
Selenium deficiency and some drugs affect deiodinase activity.
Taking T3/T4 medication distorts the calculation and its interpretation.

Calculate using this marker

Calculators where SPINA-GD (deiodinase activity) is used directly:

IPPK Conversion of T4 to active T3: SPINA-GD and the free T3/T4 ratio Open Comprehensive Thyroid Assessment Hormones, antibodies, and indices — TSHI, SPINA-GT/GD, and free T3/T4 Open

Frequently asked questions

What does a low GD show?

Reduced conversion of T4 into active T3. Often this is a sign of “low-T3 syndrome” in severe illness rather than a primary thyroid problem, so it should be interpreted in the context of the overall condition.

Are GD and T4-to-T3 conversion the same thing?

GD is a quantitative estimate of the maximum ability to carry out that conversion at the whole-body level. It reflects the total activity of the deiodinases that turn T4 into T3 in peripheral tissues.

Sources

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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