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When our physician suspects we might have a problem with our thyroid he sends us to check the TSH level in our blood. If there is a lot of TSH in the blood it means that the thyroid can't work quickly enough to process all of the TSH meaning that we have a low level of thyroxine. Having a low TSH level means the thyroid produces too much thyroxine. Those are the usual physician's assumptions. But is it possible that the problem sometimes lies in the pituitary gland which produces TSH and not in the thyroid which receives it?

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  • Welcome to Biology SE. I think this question would be better suited for Medical Sciences SE. On a side note, it is unusual for a physician to draw conclusions on the thyroid based on TSH levels without also analyzing T3/T4, so I think the premise of your question is incomplete/incorrect. – Mowgli Jan 20 at 18:42
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When a physician is studying someone for a suspected disorder related to the thyroid hormones, he/she should not only study the thyroid, but evaluate the hypothalamus-pituitary-thiroid endocrine axis and a frequent initial assumption is that the problem could be at any of these 3 levels instead of assuming the problem is in the thyroid gland.

Thyroid serum profiles include TSH, because as you said, the problem might be at a pituitary level and also this allows for further evaluation of the feedback system of the axis. Another important note is that a healthy thyroid gland will produce thyroxine proportionally to the serum levels of TSH, that is it WILL work quickly enough in response to these high levels of TSH and produce high levels of thyroxine, however a healthy axis would also lower the TSH levels after this.

To properly understand this we need to review that for this axis:

  • The hypothalamus produces the Thyrotropin-releasing hormone (TRH) in an inversely proportional manner to thyroxine blood levels. TRH estimulates the production of Thyroid-stimulating hormone (TSH) in the pituitary.

  • The pituitary produces TSH in a directly proportional manner to TRH levels and also in an inversely proportional manner to thyroxin blood levels (it has a negative feedback control independent of the hypothalamus and a positive one dependent on the hypothalamus). TSH estimulates the production of thyroxin by the thyroid.

  • The thyroid produces thyroxin and this hormone then acts in several tissues across body regulating metabolism. Thyroxin production goes down if TSH levels go down.

Now for some generic examples, if a patient has hypothiroidism signs and symptoms, and then we see a thiroid profile like this:

  • Thyroxin levels: low
  • TSH levels: high

This suggests the problem is in the thyroid gland since the pituitary is doing its job by increasing TSH to demand more thyroxin, but the thyroid is not responding at all.

Another example:

  • Thyroxin levels: low
  • TSH levels: low

The problem could be in the pituitary this time since even though thyroxin is low the pituitary isn't asking for more thyroxin by producing more TSH.

On the other hand if we suspect hyperthyroidism and we see:

  • Thyroxin levels: high
  • TSH levels: low

We suspect of a thyroid-level problem again because the pituitary is trying to slow down the production by lowering TSH, but the thyroid is independently producing more and more thyroxin (one of the first things we would suspect is a thyroid hormone producing tumor).

Another example:

  • Thyroxin levels: high
  • TSH levels: high

We could suspect that there is something wrong in the pituitary (again it could be a tumor) since TSH levels are high regardless of high thyroxin levels, the pituitary is not slowing down in a situation in which it normally would.

As you can see the disorders are produced when the feedback systems begin to be lost and the different levels of the axis become unresponsive or self-controlled.

Other strange situations would include problems at hypothalamic level or production of thyroxin by tumors outside of the thyroid, but these are never suspected in first instance since they are very rare.

A brief, but ok review of this axis can be read at Wikipedia or if you want a more robust review you can consult Greenspan's endocrinology which is what I read back in the day (:

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    Thank you for the in-depth explanation. This was really helpful. – Hinko Pih Pih Jan 20 at 20:15

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