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The Thyroid Blood Tests

Understanding what the thyroid blood tests mean helps patients understand what their doctors are seeing and why we might make certain medication adjustments.

The thyroid gland makes thyroid hormone. It mostly makes T4. The lab may show “free T4” or “total T4”. Basically, “free T4” is a better test. After T4 is released by the thyroid gland it gets converted into T3, which is the active form of the hormone. T3 and T4 fluctuate a lot, so they may not be very reliable (although I do rely on them in specific circumstances). What we tend to rely on the most is TSH. TSH is another hormone. It does not come from the thyroid gland; it comes from the pituitary gland. The pituitary gland is a little gland that sits at the bottom of the brain. It’s the “master” gland, because it controls most of the other glands in the body. The way it works with the thyroid gland is what we call a feedback loop. The pituitary gland monitors the blood to see how much thyroid hormone is in the blood. It some how knows exactly how much our body needs. If there is not enough thyroid hormone, for example, the pituitary gland will make more TSH. TSH stands for Thyroid Stimulating Hormone. It’s a feedback signal try to tell the thyroid gland that your body needs more thyroid hormone. So the TSH will always go in the opposite direction as your actual thyroid hormone level. For example, someone with low thyroid hormone will have an elevated TSH. The opposite is true if your thyroid level is too high – the pituitary will stop making TSH (suppressed TSH) so you will see a low TSH.


TSH 11.01 (high)

Free T4 0.85 (normal)

This patient has hypothyroidism (low). Even though T4 is normal the TSH is elevated.

TSH 0.05 (low)

Free T4 2.6 (high)

This patient has hyperthyroidism (high)

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