Final answer:
Increased total T3 and T4 levels during pregnancy are due to increased binding to thyroid-binding globulin caused by higher estrogen levels, leading to more thyroid hormones being carried in the bloodstream while maintaining normal free hormone levels and normal TSH.
Step-by-step explanation:
The correct option : a
The increased levels of TBG in pregnancy are due to the elevated estrogen levels, which enhance the production of TBG in the liver. This creates a higher capacity for binding T3 and T4, leading to increased total levels of these hormones while free levels remain normal, because the body maintains a tight regulation of the unbound, biologically active thyroid hormones.
Thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3), regulate the basal metabolic rate and play a vital role in development and metabolism. These hormones are produced in the thyroid gland through a process that involves the binding of TSH to thyroid follicles, the transport of iodide ions, and their conversion to iodine, which is attached to thyroid hormone precursor thyroglobulin using a peroxidase enzyme. T4 is released in greater amounts and is converted to the more active T3 in peripheral tissues.
During pregnancy, an increase in TBG due to heightened estrogen levels results in more of these thyroid hormones binding to TBG, ramping up their total levels in the bloodstream. However, the free, active forms of T3 and T4, which cross cell membranes and exert effects on metabolism and growth, are tightly regulated. So while total levels rise, the proportion of active hormone does not change significantly, which is why TSH remains within the normal range.