Thyroid disease can lead to abnormal uterine bleeding. This type of uterine bleeding can make a woman niddah.
You can perform a hefsek taharah as soon as you are able to get an acceptable bedikah. Remember that you can wash first. Be sure to bring any questionable colors to a halachic authority for guidance, mentioning your medical condition and your difficulty getting to mikveh.
Until your condition improves, in addition to the hefsek taharah, you should perform only one bedikah on day one, one on day seven, and one on an intermediate day of the clean days. Try to time the bedikot for when you have less staining. Please get back to us, our hotline, or a local halachic authority if these measures do not suffice to get you to the mikveh.
Once you have immersed, be careful to take all the relevant precautions to avoid becoming niddah from staining.
We wish you a refuah sheleimah!