Under stress, the body slows down by decreasing the amount of raw material T4 that is converted to the active thyroid hormone T3, while increasing the amount that is converted to the inactive RT3. It has been shown that during fasting, the T3 level in the bloodstream can drop by 50% with the RT3 going up by 50%. Since T3 is an extremely active thyroid hormone and since RT3 has no thyroid hormone activity, it is obvious that this shunting process can greatly affect the amount of physiologically active thyroid hormone at the level of the active site. Studies have shown that the metabolic rate drops during these same conditions of fasting.
Incidentally, it has been shown that some of the highest levels of RT3 found in man are in newborn babies. Cord samples of blood taken from the umbilical cord at the time of birth often show elevated levels of RT3 and low levels of T3 (which begin to increase soon after birth). This may be a survival mechanism to help the baby to conserve as much energy as possible and get a foothold in this world. I have often wondered if this is why babies spend so much time sleeping. Basically, all they do is eat, sleep, and gain weight and can often be on the irritable side. A little hunger seems to be so much more painful for them, and their hunger-pang screams seem so much more urgent and desperate, as if they’re faced with a life-threatening situation. But after they are fed they are extremely content and satisfied. As we will discuss next, it seems that the conservation mode is triggered when the body perceives a threat that there may be insufficient resources to meet apparent challenges. The lower the resources, the more desperate the situation. Probably few of us can think of an animal or organism with fewer resources or that is any more vulnerable than a human baby.
When the body is faced with stress or starvation, and T4 to T3 conversion decreases, the cells of the body slow down, so the body temperature drops. When the temperature drops, many of the body’s enzymes do not function as well.