Please note: I'd like to preface this question by stating that this is neither homework nor me seeking medical advice. I am simply trying to understand the biological, physiological root etiology behind various thyroid-related structures.

This question shows research and is not a duplicate.

My undertanding of the pituitary gland is that it will produce TSH when it senses total T4 (not free T4) is low. So if both total T4 and TSH are low, this implies two things:

  • There is some underlying reason why T4 is low; but moreover...
  • There is some other (or related) reason as to why TSH is low

In all the literature I can find, the main antagonist in this scenario is a malfunctioning pituitary gland.

So I ask: if the pituitary gland is deemed functional and non-diseased, are there any other root causes for both a low total T4 and a low TSH? Biologicaly/physiologically, how do they work to produce this low level of hormone production?

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    $\begingroup$ Are you sure such a condition exists? Can you link to it? Also, are you sure about your total T4 statement? $\endgroup$ Sep 11 '15 at 8:31
  • $\begingroup$ Thanks @anongoodnurse (+1) - here is an article showing that a low TSH and a low/normal T4 are attributed to pituitary disorder. I can provide other links as well. $\endgroup$
    – smeeb
    Sep 11 '15 at 12:53
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    $\begingroup$ Yes, I'm sure it can happen with a pituitary condition, but you stipulated in your scenario that the pituitary was healthy. I'm asking you if you know that condition to exist, and can you link to it? $\endgroup$ Sep 11 '15 at 18:23
  • $\begingroup$ Thanks again for verifying @anongoodnurse (+1) - if you scroll to the bottom of that link, you'll see a table where it shows that low TSH and low T4 can be caused by two categories of conditions: (1) "rare pituitary (secondary) hypothyroidism" (which like you understand, we're assuming to not be the case here as the pituitary is healthy), and (2) "Non-thyroidal illness". $\endgroup$
    – smeeb
    Sep 11 '15 at 19:00
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    $\begingroup$ When you ask a question, it's much easier to summarize the part you have a question about rather than make a user read the whole paper just to understand what you question. That's why I asked for more information in my first comment. $\endgroup$ Sep 11 '15 at 20:09

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