One thing that is consistent in this forum is that there is no consistency. I know that is a paradox and I will explain. This will take some time and I may have to spread it over several posts and I will reference it after the final post. People with B12 deficiency often suffer exhaustion, even when they test negative for anaemia. How can this be? What is the causal mechanism that is robbing them of energy? To answer this question it is necessary to look at a bit of biochemistry.
The speed that the body runs at (energy available) is governed by the amount of active thyroxine in the blood. The more thyroxine the faster the body runs. This is regulated by the pituitary gland in the brain. As the amount of thyroxine in the blood falls, the pituitary releases Thyroid Stimulating Hormone (TSH) into the blood-stream. This stimulates the Thyroid gland to produce thyroxine - the go faster hormone.
The pituitary gland needs certain biochemical resources to make this TSH. These are: Protein (not usually a problem), Magnesium (often inadequate in European diets), Zinc (often low in processed foods) and Vitamin B12. If anything is missing - or low - TSH cannot be made. With low TSH the thyroid will not make thyroxine and energy levels in the body will drop. This has nothing to do with anaemia. It is the driving signals that regulate energy levels. An assessment of blood composition will not reveal the problem - because it is not the blood that is the problem.
Tiredness is a very very very common problem for doctors. They have invented an acronym for it for their notes: TATT (Tired All The Time). When they test for thyroid function, they do not test for thyroxine levels directly. They test for TSH levels. Under normal circumstances, if thyroxine levels are low TSH levels will be high as the pituitary gland tries to signal to the thyroid gland to produce more thyroxine. This is the yardstick that they use to measure thyroid adequacy. (Low TSH thyroid ok. High TSH thyroid dysfunctional)
However - if circumstances are not normal, this system can deceive the doctor. If thyroxine levels are low the pituitary will try to make TSH. If it does not have the necessary chemical wherewithal to produce the TSH it is possible that there could be low thyroxine AND low TSH. As the doctor does not test thyroxine level directly but relies on the TSH scores this would give a false picture of thyroxine adequacy. As B12 is critical to the production of TSH, it is perhaps not surprising that hypothyroidism and B12 deficiency go hand in hand in many cases.
The take-away message from this first section of the story is that you need in your diet, protein, zinc, magnesium and vitamin B12 to regulate the levels of thyroxine hormone and consequent energy levels. This is not the end of the story. See subsequent posts.