TSH is Thyroid Stimulating Hormone, also called thyrotrophin. The name is a bit misleading because it seems like TSH is a thyroid hormone, but it is not. TSH is made by a special part of the brain called pituitary gland. Pituitary regulates many endocrine glands, including ovaries, testicles, adrenal glands and, of course, the thyroid gland.
As the name suggests, TSH stimulates the thyroid gland into making more thyroid hormones. When the body needs more thyroid, it makes more TSH. And if you don’t need more thyroid hormones then you produce less TSH.
It’s sort of like a whip for a horse. If a horse is running well, you don’t need to use a whip. But if the horse slows down, the whip will make it run faster.
If the thyroid gland is working well, it needs only minimal stimulation. As a result, TSH level is usually quite low. But if the thyroid starts slowing down and does not do a good job, TSH levels will go up, to stimulate the thyroid and make it work harder.
This system is called “negative feedback.” High thyroid hormone levels will drive TSH down, low thyroid hormones will make it go up. So, if you see someone with low TSH you presume that her thyroid is fine. And if you see someone with high TSH, you make a conclusion that their thyroid is low.
This is why doctors measure TSH levels to diagnose patients with hypothyroidism. And, in general, it works out. Tens of millions of people have high TSH, they are being diagnosed and treated.
But, like with every rule, there are exemptions. It is true that most patients with low thyroid will have high TSH. What is not true is that ONLY people with high TSH have hypothyroidism.
Which is why relying on TSH alone is wrong for several reasons.
First, what is high? In most labs so called “normal” TSH is between 0.27 to 4.2. Sometimes 4.5. The units are not important right now, I just want to explain the idea. This “normal” range is also called “reference range”. And, as you see, this is a pretty big range, from 0.2 to 4.2.
I have another lecture where I explain how reference range is established. For now, I’ll just tell you that a reference range includes 95% of seemingly normal people. This is important. Because hypothyroid is so common, it affects tens of millions of people, and many people have low level hypothyroidism with mild symptoms. They don’t consider themselves sick and no one else does either.
They are “seemingly normal” and naturally they are included into the reference or “normal” range.
That’s why many experts consider TSH normal when it is less than 2.0 or even less than 1.5 So TSH of 4.0, though still in the “normal” range, is too high and the person likely has hypothyroidism. That’s why using TSH alone is not a good idea.
Plus, as I have already pointed out, there are always exceptions. I have seen many people with “normal” or even “low” TSH, yet they were obviously hypothyroid.
It’s OK to check TSH, but it should not be the only test. It should be part of a panel of thyroid tests, which include T4, free T4, T3, free T3, reverse T3, and often antibodies against TPO and thyroglobulin.
I have a complete report about proper thyroid testing which you can download free from my site.