The low TSH suggests the 200mcg synthroid is too much - with the borderline low T3 would consider a trial of combination therapy --- first check an am cortisol to screen for adrenal --- then consider decreasing synthroid to 150mcg and ADDING cytomel 5mcg 2x/day -- you will likely need to see a specialist to start such a protocol.
Your TSH is low because you're taking oral thyroid medication. That's fine - only most of the doctors I've seen then think I'm hypER and reduce my meds to the point that I'm so hypo I can't function. That's the problem with dosing only by TSH, which it looks like your doctor isn't doing, fortunately. However, you say your Free T3 is very low in the range (you didn't post the range) - I've read where your T3 should be in the upper 1/3 of the range, at least, for you to feel well. Have you tried Armour, which has T3 and other thyroid hormones, or even adding some Cytomel (synthetic T3)?
Also, salt cravings are known to indicate adrenal problems. Have you had that checked?
Just wondering if they've done some of the more basic tests like a CBC and Chem 26? A Chem 26, is really inexpensive (so is CBC)and includes sodium, chloride etc..., the electrolytes that can indicate adrenal-mineral regulation. It also has kidney and liver function readings on it.
You might also talk to your Dr. about getting cortisol levels checked. Some think just one blood reading is needed but studies show that cortisol needs checked at least at both 8:00AM and 12:00Midn. levels, because it helps better to see your pattern (rhythm) of adrenal function.
I haven't had any other testing except TSH & T3 ever. I'll ask about it. Also I'm taking an over the counter cortisol blocker which has helped me feel better, although weight loss continues to allude me. What is the connection between cotisol and my test results? Should I stop taking the suppiment?
Normal range on test indications 230-420 as normal. So 256 is not even mid-range.
The diet supplements put out for losing weight from what I understand, are not proven to block cortisol at all but they claim to block "the effects" of cortisol. If they really did block cortisol production, this would potentially cause adrenal suppression in people taking them and would definately be a bad idea for people with adrenal fatigue/exhaustion or true adrenal insufficiency.
As you mentioned about pituitary function, it does seem that some people have what might be called a sub-clinical type of hypo-pituitarism (underfunctioning) and the gland doesn't send enough TSH. In these people, a low TSH (even clinically low), should mean there is too much thyroid hormone, when there is actually not yet enough! With your TSH as low as it is getting, your Free T-3 should read higher than that ("256"). In a range of "230 to 420", 325 would be middle but you are in the lower half. Many Endocrinologists I've read, stated that the thyroid hormones should be above middle range for most patients to feel better.
You may not have true hypo-pituitarism but may be one of the many people who's Dr. needs to ignore TSH and keep tabs on your hormone replacement progress, by testing the actual thyroid hormone levels. Many on this forum have testified to their TSH level looking hyper, when in reality, their thyroid hormones were just reaching optimal range. It's POSSIBLE, you need a higher dose to feel better.