Thyroid's secretory capacity is the maximum stimulated amount of thyroxine that the thyroid can produce in a given time-unit.
How to determine GT
Experimentally, GT can be determined by stimulating the thyroid with a high thyrotropin concentration and measuring its output in terms of T4 production, or by measuring the serum concentration of protein-bound iodine-131 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 or K41: Dissociation constant T4-TBG
K42: Dissociation constant T4-TBPA
DT: EC50 for TSH
Specific secretory capacity
The ratio of SPINA-GT and thyroid volume VT i.e. or is referred to as specific thyroid capacity.
experiments with Monte Carlo simulations demonstrated that both SPINA-GT and SPINA-GD can be estimated with sufficient reliability, even if laboratory assays have limited accuracy. This was confirmed by longitudinal in vivo studies that showed that GT has lower intraindividual variation than TSH, FT4 or FT3.
Clinical utility
In clinical trials SPINA-GT was significantly elevated in patients suffering from Graves' disease and toxic adenoma compared to normal subjects. It is also elevated in diffuse and nodular goiters, and reduced in untreated autoimmune thyroiditis. In patients with toxic adenoma it has higher specificity and positive likelihood ratio for diagnosis of thyrotoxicosis than serum concentrations of thyrotropin, free T4 or free T3. GT's specificity is also high in thyroid disorders of secondary or tertiary origin.
Pathophysiological and therapeutic implications
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 syndrome SPINA-GT may be temporarily elevated. In chronic NTIS as well as in certain non-critical chronic diseases, e.g. chronic fatigue syndrome or asthma SPINA-GT ist slightly reduced. In women, therapy with Metformin results in increased SPINA-GT, in parallel to improved insulin sensitivity. This observation was reproducible in men with hypogonadism, but not in men with normal testosterone concentrations, so that the described effect seems to depend on an interaction of metformin with sex hormones. In hyperthyroid men both SPINA-GT and SPINA-GD negatively correlate to erectile function, intercourse satisfaction, orgasmic function and sexual desire. Likewise, in women suffering from thyrotoxicosis elevated thyroid's secretory capacity predicts depression and sexual dysfunction. Conversely, in androgen-deficient men with concomitant autoimmune thyroiditis, substitution therapy with testosterone leads to a decrease in thyroid autoantibody titres and an increase in SPINA-GT. In patients with autoimmune thyroiditis a gluten-free diet results in increased SPINA-GT. Statin therapy has the same effect, but only if supply with vitamin D is sufficient. Accordingly, substitution therapy with 25-hydroxyvitamin D leads to rising secretory capacity. This effect is potentiated by substitution therapy with selenomethionine. The effects of vitamin D and selenomethionine are attenuated in hyperprolactinaemia, suggesting an inhibitory effect of prolactin. On the other hand, men treated with spironolactone are faced with decreasing SPINA-GT. It has, therefore, been concluded that spironolactone may aggravate thyroid autoimmunity in men. A study in euthyroid subjects with structural heart disease found that SPINA-GT predicts the risk of malignant arrhythmia including ventricular fibrillation and ventricular tachycardia. This applies to both incidence and event-free survival. Specific secretory capacity is reduced in obesity and autoimmune thyroiditis.
