Can you overdose on Kelp supplements?

Just wondering if anyone knows about overdosing on Kelp supplements with a Hypothyroid?
I have been trying to balance my thyroid for 6 months using 660mg/day of kelp. I have been getting a blood test monthly but it seems to go up and down, somtimes being in balance and other times being out of balance (hypo). My doctor says I may be taking too much kelp and hence it goes out of balance the other way, causing a more hypo condition. Another possible factor might be that I eat seaweed regularly as I live in Japan. But Hypothyroidism is not as common in Japan but is still present.

Maybe I have an absorption problem??

Anyway, any feedback would be greatly appreciated!
Thanks,
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