TSH stands for Thyroid Stimulating Hormone. It is produced by the pituitary gland in the brain. The function of TSH is to stimulate the thyroid gland, to make it work harder and produce more thyroid hormones.
If the thyroid gland is doing a good job and makes enough hormones, it does not need any extra stimulation, so the level of TSH will be low. But if there are not enough thyroid hormones, pituitary will produce more TSH so the thyroid gland will make more of them.
In other words, when hormones go down TSH goes up. And when hormones go up TSH goes down. That’s how it works in most people and that’s how doctors diagnose hypothyroidism, when TSH becomes too high.
The question is – what is normal and what is too high?
Each healthy person has a certain level of TSH that is normal for him or her. Just like each healthy person has a certain height that is normal for him or her, or weight or shoe size or eye color.
The problem is, we don’t know what level of TSH is normal for a particular person. Let me repeat that. We do not know what level of TSH is normal for you or for me or for anybody.
The best we can do Is check the level in a few thousand people and see where most of them are. Each person would have a certain level, one person might have 3, another 0.5, another 2, yet another 1 or 4 or 5 or 10. Then we calculate the range that covers 95% of the people and we call it “normal range.”
In most labs, this “normal range” is between 0.5 and 4.5.
If we do the same with the height, we will find that 95% of people are between 5 and 6 feet, so we can say that normal adult height is between 5 feet and 6 feet.