Great replies here. You're definitely in the right place.
Agreed that your Free T3 must be checked. I have no doubt you are low in T3 due to nutrient deficiencies. And yes, it is therefore most likely your symptoms are thyroid related in that the thyroid is not getting the nutrients that are vital to its functions and conversions. Without such, the body begins to dysfunction on many levels and will continue to do so until the deficiencies are replenished to optimal levels.
It is so terrible how conventional medicine perpetuates disorders by failing to recognize hypothyroidism as a more likely cause of dysfunction in the body. But conventional medicine is designed this way. Their testings are lame and unreliable, yet they continue to use them as we get sicker and are given a label of this or that in order to promote more testings and drugs that often worsen thyroid function and make us even sicker.
The connection has been documented regarding Guillain–Barré syndrome and hypothyroidism, but the report at the link below fails to realize this is far more common because the syndrome (group of symptoms) is caused by hypothyroidism that conventional medicine's inadequate testing methods don't diagnose:
"We report a case of Guillain–Barré syndrome associated with Hashimoto's thyroiditis which is a chronic inflammatory autoimmune disease well known and one of the most common causes of primary hypothyroidism. ... Immunoglobulins or plasmapheresis are the ideal treatment Guillain–Barre syndrome." Jan 19, 2016
It could very well be that all that is needed for you to regain health and muscle strength are the nutrients that will facilitate thyroid function that is vital to well-being and to life itself. If you are deficient in Vitamin D, I guarantee you -- you are deficient in a number of other nutrients. Example, without adequate magnesium, Vitamin D doesn't get synthesized. I would venture to say you are also deficient in magnesium. Conventional doctors know nothing of such things.
"Simply by increasing magnesium intake, we may decrease mortality related to vitamin D deficiency — without taking any extra vitamin D. But beyond vitamin D's relationship to magnesium is magnesium's relationship to calcium. And in some ways, these two minerals have opposite effects."
Please see a doctor who understands how the body works and who can help you correct your deficiencies and address what those deficiencies have done to lower your thyroid function. Conventional doctors have no clue about these things. They instead spent most of their medical school training learning about pharmaceuticals directed by Big Pharma for them to use for a myriad of Syndromes and Disorders that are, for the most part, symptoms of hypothyroidism.
Hope this helps!