When the inactive T4 thyroxine reaches the liver, it must be converted to the active form T3. To achieve this the mineral Selenium is needed. Selenium is not very common in European soils and food-deficiencies are not unusual. If there is a selenium deficiency the liver converts the T4 to REVERSE T3. This is another inactive form of thyroxine. Therefore there may be plenty of thyroxine in the bloodstream but much of it may be functionally worthless.
When active T3 is released into the bloodstream it must bind to the nuclear walls of the cells to signal to go faster. To properly bind and send the signal it needs Vitamins A and D. Deficiency in Vitamin A is rare, but deficiency of Vitamin D is not. Reckless and poorly informed advice from dermatologists has caused concern among many people about the effect of Ultraviolet light on skin. Vitamin D is made from cholesterol in the skin by the action of Ultraviolet light. High factor sun-blocks and avoidance of sunshine can and will cause Vitamin D deficiency. As with many other issues moderation is the key. The takeaway message from this section is to eat a decent non-processed-food diet and experience moderate sunshine on bare skin. (Do NOT burn). Selenium can be obtained by circa 6 Brazil nuts per day.