Vitamin B12 deficiency is often associated with the most common form of hypothyroidism, autoimmune thyroid disease. In India, more than 50% of hypothyroid patients were found to be B12 deficient, but this is in a population with a much higher vegan/vegetarian diet compared to Taiwan that reported B12 deficiency in a little over 6% of those tested. The studies also don't always use the same laboratory cut-off levels for diagnosis, which is why the prevalence of B12 deficiency is reported over a wide range of 6.3% - 55%. Regardless of this, many experts in the field recommend screening for B12 deficiency in hypothyroid patients at diagnosis and periodically thereafter. People who have an under-active thyroid gland frequently complain of a variety of symptoms, but numbness, tingling and difficulty swallowing were most often reported by hypothyroid patients with vitamin B12 deficiency.