Which feature best defines hyperinsulinemia that can affect sex hormone binding globulin?

Hyperinsulinemia is characterized by an excessive level of insulin in the blood. This condition can lead to a variety of physiological changes in the body, one of which is its effect on sex hormone binding globulin (SHBG). Elevated insulin levels suppress the production of SHBG, resulting in decreased SHBG availability to bind sex hormones such as testosterone and estrogen. As a consequence of lower SHBG levels, there can be an increase in free or bioactive sex hormones, which may have significant implications for reproductive health and metabolic functions.

The other options provided do not encapsulate the primary feature defining hyperinsulinemia. Decreased blood glucose levels, for example, might occur with hypoglycemia or other conditions but do not directly relate to hyperinsulinemia, which is marked by high insulin levels. Similarly, while high cortisol production and low thyroid function can impact metabolism and hormone levels, they are not defining characteristics of hyperinsulinemia itself. The essence of hyperinsulinemia lies within the context of elevated insulin levels and their broader systemic effects.