I would greatly appreciate having access to this article. Thanks!
“Her thyroid function tests were all within the normal range although both TSH and free T4 were in the low normal range...We discuss the question of having hypothyroid symptoms with normal thyroid function tests, should we treat it and how to evaluate the clinical and laboratory response to treatment in this patient.”
Having read that paper - quickly, and thanks to jimh111 - I reckon that the overall view they express is spot on.
Of course the pituitary's response can be, in effect, blunted. Of course it can produce too little TSH. Every other part of our bodies can be impaired, why not the pituitary? And, surely, for every person who has sufficient impairment to be classed hypopituitary, and so identified as suffering from overt central hypothyroidism, there will be several who are somewhere between "normal" and overt?
If we consider adults only, who originally had full pituitary function, this is inevitably the case. It can take years for the hypothyroidism to be diagnosed. Followed by years of dismissal. Only eventually, as in the case, for a minority to be diagnosed at all. (Diagnosis seems likely only if the severity progresses.) There simply must be more people in this in-between state than diagnosed overtly centrally hypothyroid.
Why the deterioration in pituitary function? I suggest physical injury and auto-immune pituitary disease as two possibilities.
I think the primary cause of deterioration of pituitary function (actually just the throtrope) is a down-regulated axis which can be caused by a period of hyperthyroidism which the patient may not notice.
Do you suggest that this period of hyperthyroidism could be due to Hashimoto's, as in flares, or some other cause?
Yes, usually thyroid autoimmunity. There can be other causes less common causes such as head trauma, depression, strict dieting and concurrent severe illness.
It can also be caused by patients taking TSH suppressive doses of thyroid hormone, they end up in a one-way street. This is why I always suggest patients try to recover without suppressing their TSH if they can.
The current belief is no certain advice (in the sense of being "blanket advice") can be made for any patient, based on all-consuming assumptions generally aimed at everyone. This generalisation also creeps in in TUK advice to patients. For everyone, the only solution is to try out individual regimes to see if they benefit or not. For example, though suppressed TSH on therapy may be disadvantageous for some patients, for others it is essential in order to attain sensible levels of FT4 and FT3. There may still be possible problems even here, but if the choice is between suppressed TSH and decent QoL from adequate FT4/FT3 levels, the trade-off choice is obvious. The great unsolvable problem is that when healthy, the thyroid hormone levels are never measured, so that when disease strikes, there is no general target to aim at to try to restore levels back as close to health as possible.
I would agree, it's difficult to know whether TSH is very low because of high hormone levels, insufficient secretion or individual set point. I dislike the term 'suppressed' as it is highly loaded, TSH is not always low due to suppression. 'very low TSH' would seem to be a better descriptive term that does not presume suppression.
Subclinical hypothyroidism
hypothyroidism - Subclinical 
Subclinical hypothyroidism 
