Another Interesting Case

I saw this interesting case that has me salivating all over it. No it's not because she's hot; but rather like many academics out there, I'm pretty much a nerd and a sucker for interesting cases. I'm in the midst of working her up but I have a pretty strong hunch.

This was a 54 year old woman referred to me for 'hypothyroidism'. Was found to have an elevated TSH of 6.2 (normal 0.5-4.5). Her GP started her on Levothyroxine and was puzzled when her follow up labs yielded a TSH of 7.2 and a free T4 of 2.3 (normal 0.5-1.5). The labs remained about the same after the medications were subsequently stopped.

Her history is otherwise significant only for hyperlipidemia, mild hypertension and a history of hysterectomy for fibroids. Family history is negative for any thyroid issues though diabetes is somewhat prevalent in her family. Aside from some rare palpitations, she feels well.

How would you proceed working this patient up? What is your preliminary diagnosis?