Experimentally, G 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 iodine-131 after administration of radioiodine. These approaches are, however, costly and accompanied by significant exposure to radiation.
In vivo, G can also be estimated from equilibrium levels of TSH and T4 or free T4. In this case it is calculated with
or
[TSH]: Serum thyrotropin concentration (in mIU/L or   μIU/mL)<br />
[FT4]: Serum free T4 concentration (in pmol/L)<br />
[TT4]: Serum total T4 concentration (in nmol/L)<br />
: Theoretical (apparent) secretory capacity (SPINA-GT)<br />
: Dilution factor for T4 (reciprocal of apparent volume of distribution, 0.1 L)<br />
: Clearance exponent for T4 (1.1e-6 sec), i. e., reaction rate constant for degradation<br /> 
K: Binding constant T4-TBG (2e10 L/mol)<br />
K: Binding constant T4-TBPA (2e8 L/mol)<br />
D: EC for TSH (2.75 mU/L)
The method is based on mathematical models of thyroid homeostasis. Calculating the secretory capacity with one of these equations is an inverse problem. Therefore, certain conditions (e.g. stationarity) have to be fulfilled to deliver a reliable result.