Thyroid's secretory capacity

How to determine GT

Experimentally, GT can be determined by stimulating the thyroid with a high thyrotropin concentration (e.g. by means of rhTSH, i.e. recombinant human thyrotropin) and measuring its output in terms of T4 production, or by measuring the serum concentration of protein-bound ¹³¹I after administration of radioiodine.

In vivo, GT can also be estimated from equilibrium levels of TSH and T4 or free T4. In this case it is calculated with

G ^ T = β T ( D T + [ T S H ] ) ( 1 + K 41 [ T B G ] + K 42 [ T B P A ] ) [ F T 4 ] α T [ T S H ]

or

G ^ T = β T ( D T + [ T S H ] ) [ T T 4 ] α T [ T S H ]

G ^ T : Theoretical (apparent) secretory capacity (SPINA-GT)
α T : Dilution factor for T4 (reciprocal of apparent volume of distribution, 0.1 l⁻¹)
β T : Clearance exponent for T4 (1.1e-6 sec⁻¹)
K₄₁: Dissociation constant T4-TBG (2e10 l/mol)
K₄₂: Dissociation constant T4-TBPA (2e8 l/mol)
D_T: EC₅₀ for TSH (2.75 mU/l)

The ratio of SPINA-GT and thyroid volume (as determined by ultrasonography) is referred to as specific thyroid capacity (SPINA-GTs).

Reference Range

The equations and their parameters are calibrated for adult humans with a body mass of 70 kg and a plasma volume of ca. 2.5 l.

Clinical significance

SPINA-GT is elevated in primary hyperthyroidism and reduced in primary hypothyroidism, and it has been observed to correlate with thyroid volume. In longitudinal evaluation GT shows lower intraindividual variation (i.e. higher reliability) than TSH, FT4 or FT3.

Correlation of SPINA-GT with creatinine clearance suggested a negative influence of uremic toxins on thyroid biology. In the initial phase of major non-thyroidal illness SPINA-GT may be temporarily elevated.

Specific secretory capacity (SPINA-GTs) is reduced in obesity and autoimmune thyroiditis.