Vitamin B12 deficiency can cause low thyroid as one of its many manifestations.   Of course, B12 deficiency isn't properly tested for or treated in Canada.

A European doctor told me that the definitive way to check for low thyroid function is to check basal body temperature in the armpit upon waking after sleeping through the night and, importantly, before even sitting up to get out of bed.   He claimed that a lot of cases of hypothyroidism missed by the standard thyroid blood tests can be picked up using basal body temperature tests, especially in the early stages.   When the thyroid is underactive, the temperature is lower than ideal.