A
B12 level of 278 is on the lower side and can still explain ongoing weakness, especially with IBD and Hashimoto's affecting how well you absorb nutrients. The supplement you're already on is a reasonable starting option, but your continuing weakness suggests the form or length of treatment needs a fresh look.
Next Steps
Please ask your physician or gastroenterologist whether a different
B12 formulation, such as methylcobalamin or an injectable form, would suit you better, and how long to continue it based on a repeat B12 level.
Health Tips
Please don't change or stop the supplement on your own, since your IBD and
thyroid condition both affect how long you may actually need it.