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Hello from JustAnswer.
There are a couple issues in this situation.
It i appropriate to be trying to keep the TSH below 0.15 in this situation, but if the TSH is too low, most doctors would typically be more gradual in reducing the dose of the Synthroid, rather than reducing the dose by half. Taking a 150 mcg dose each day would be a reasonable dose.
Whether it would be appropriate to request another scan would depend upon when the last scan was done. Certainly, if the dose of Synthroid that has been achieving the desired level of TSH is now too much Synthroid, it is a reasonable concern that there may be some return of thyroid tissue. However, if the most recent scan was done within the last 3-6 months, it would be reasonable to first consider other factors that may affect thyroid hormone metabolism, particularly kidney function or conditions that secondarily affect kidney function.
So, if the last scan was recent and kidney function was not checked when the TSH was checked, that would be the next step. If kidney function was checked with a BUN and creatinine, if you provide the tests of kidney function in this test and any previous test that you have, then I can comment upon these values.
If I can provide any additional information, please let me know.