How long before getting your blood drawn did you take your thyroid medication? It takes an hour for most of the T4 to get in the blood. This will cause a spike in blood T4 levels for 3 - 4 hours after absorption. Then about 2 hours after taking your meds your get a secondary conversion spike where a lot of the excess T4 running about is grabbed and sent into T3 conversion. You tend to have higher than normal T3 levels 3-5 hours after taking your meds. Rule of thumb for taking your thyroid meds on the day of a blood draw:
1) If your blood draw is early morning, wait until after your blood is drawn.
2) If your blood draw is in the afternoon, take your T4 supplement at least six hours prior to your blood draw.
Tigger is she monitoring your Ft3 levels as well? Having a spiraling lower TSH with decreasing dosage points to one of two things. Your T4 to T3 conversion may be in over drive or your pituitary gland is not functioning as it should. Has your MD checked your adrenals (these affect the rate of T4 to T3 conversion) and your pituitary gland (This gland tells the thyroid when to make more meds as well as the adrenal glands when to make more hormones as well. I like to imagine the pituitary gland as the hormonal over seerer.)? Both are important to keep an eye on if you have been hypothyroid for a long time. Long term untreated hypothyroidism has been known to cause the adrenals to become fatigued or shut down. In the case of Hashimoto's and Graves, AI attack can be polyglandular. All in all just watching the TSH and Ft4 and tweeking your T4 dosage may not be enough if you are a complex case.
Just my non-MD opinion. But my first IM and Endo put me off long enough on treatment that my adrenals shut down and my pituitary gland just doesn't work like it is supposed too. I have lazy glands. I wish I could take a vacation like they did. Early retirement.. sounds good, huh?