why doesn't hypothyroidism cause growth hormone excess?

[the guy]

In the case of hypothyroidism (at least those cases which result from iodine-deficiency) I am confused as to why it lowers levels of growth hormone.

I would expect, since there is a lack of iodine and therefore thyroid hormone, that thyroid stimulating hormone levels would rise. But for this to happen there would have to be lowered levels of somatostatin which would, in turn, result in high levels of growth hormone.

This is is evidently not the case though, since iodine-deficiency induced hypothyroidism is shown to cause stunted growth.

Can someone please help me explain this?

[CharonY]

Disclaimer: this is not my field of expertise and only have tangential knowledge on this topic.

 

From what I recall interaction betwen the hypothalamic-pituitary-thyroid axis and the GH=IFG-I system is rather complex and not fully understood on the mechanistic level. I.e. there is more evidence in terms of phenotypes rather than in the underlying regulatory circuits.

As such there are conflicting models that try to consolidate these effects.

 

That being said, I do not recall that TSH levels would lead to reduction of GHIH. The only model that I recall is that GH may promote T4 to T3 conversion and via that regulatory circuit (as T3 is more active).

Also there has been indications that GH may influence TSH dynamics, but evidence is a bit shaky (often the data is based on GH replacement studies and additional factors may play in).

 

For the opposite side (i.e. thyroid affecting GH-IGF-I axis) there is even less data (that I know of). One thing to note is that the stunted growth is not exclusively due to effects on GH but there is also direct effects of the thyroid horomones on bone growth, for example.

 

One of the few studies that I recall (but I am not an expert in this field) have shown in mice that especially IGF-I is affected, and to some extent also GH. But again, I am unaware of studies showing in detail the molecular interactions.

Edited August 6, 2013 by CharonY