What has been your TSH levels previously? TSH fluctuates greatly (as much as 75% over the course of a day) and is affected by many things, other than thyroid hormones; that's why it's so important to have the actual thyroid hormone levels tested.
Could you have had your testing done at different times of the day? TSH tends to be highest in the morning and lower later in the day.
Or taken medication prior to labs, where maybe you hadn't before or something like that? Taking medication prior to labs can affect the numbers, too.
In addition, our bodies change over time. Did you lose weight, have you been under stress, etc?
It's important to note that many of us have low or suppressed TSH when we're on adequate dosages of thyroid hormones. It doesn't hurt anything, because TSH is only an indicator of thyroid status and a poor one, at that; it's not a thyroid hormone. Unfortunately, too many doctors panic when they see a low TSH and automatically assume the patient is hyper; if you didn't have any hyper symptoms, you weren't hyper.
That TSH doesn't fit with your symptoms, so, at this point, I can only assume that your doctor is lowering your dosage needlessly, based on the TSH levels. This is causing you to go hypo.
My strong recommendation is to find a doctor that will test Free T4 and Free T3. Most doctors that test, and adjust medication, based only on TSH tend to keep their patients ill most of the time.
Free T4 and Free T3 are the actual thyroid hormones, whereas TSH is a pituitary hormone that neither causes nor alleviates symptoms. TSH should not be used as the sole basis for diagnosis or adjusting medications.
So you didn't have thyroid cancer? Have you never had Free T4 and/or Free T3? Your dosage has always been adjusted by the TSH level?
Your original post said you recently had a high TSH test; I'm sorry, I'm not seeing any high TSH levels; the ones you have posted above are all very low unless it was the one in early Feb that was high. We need to know what it actually was.
Do you know if you've ever been tested for thyroid antibodies?
You'll need to ask your doctor to order Free T4 and Free T3 at the very least. If s/he refuses to do so, I'd run, don't walk to find a different doctor. Also, if you've never been tested for thyroid antibodies, ask for those tests, as well. Those are Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TgAb). Those will determine if you have Hashimoto's.
The first things we need, however, is that latest TSH result.
Yes, if you haven't had Free T4 and Free T3 tested you need to ask your doctor to order them. If you've had them tested, please post the results, along with their corresponding reference ranges, since ranges vary from lab to lab and have to come from your own report.
For what reason did you have the subtotal thyroidectomy? How much of your thyroid was left? Where you started on Synthroid right after the surgery and what dosage of Synthroid have you been on?
Do you know if you have an autoimmune thyroid condition?