Thyroid lab tests explained in under 150 words
Q. I have been trying to figure out the two basic thyroid lab tests, TSH and T4. If you have a high TSH and a low T4 does that mean that the pituitary gland is going crazy to reach homeostasis but the thyroid is not responding? And inversely, if the T4 is high and the TSH is low does that mean for some reason the thyroid is working overtime due to a disease like Graves disease, and the pituitary is trying to compensate by not producing TSH?
A. Yes! That’s exactly right. When the two (TSH and T4) are opposite of each other – high T4/low TSH or low T4/high TSH – that means that the problem is intrinsic to the thyroid gland (Graves disease or Hashimoto thyroiditis, for example) and the pituitary is trying to control the thyroid by producing more or less TSH. Those are the most common types of thyroid disease – those that are intrinsic, or primary to the thyroid gland itself.
On the other hand, if both TSH and T4 are either low or high – high T4/high TSH or low T4/low TSH – that means that the process is being driven by TSH. Either there’s a pituitary adenoma making a ton of TSH, or the pituitary is not working well for whatever reason (it’s been radiated, or has undergone necrosis) and it’s not making enough TSH.
Image credit: akay (http://www.flickr.com/photos/akay/245002004/), under cc license.
