Serum B12 measures the B12 that circulates outside of the cells and not what is in the cells
Packed with electrolytes to support hydration, probiotics to help promote a healthy gut and digestive enzymes to help your digestion, a nutritionally-complete protein shake is a lighter, but also a very effective means to increase your nutritional intake
Now the other thing I think is important is also getting a full thyroid panel

remain upright for at least 23 hours Limit alcohol consumption, which can irritate the gastrointestinal tract Consider keeping a food diary to identify specific triggers Over-the-counter remedies may provide symptomatic relief: Simeticone (an anti-foaming agent, not an antacid) can help reduce gas and bloating Ginger may soothe digestive discomfort Peppermint tea might help some people, but use with caution as it can worsen GORD symptoms by relaxing the lower oesophageal sphincter Probiotics might support healthy gut flora, though evidence is limited Alginate-containing preparations or antacids may help if heartburn is also present When to seek medical advice : Contact your GP or diabetes specialist nurse if sulfur burps are persistent, severe, or accompanied by concerning symptoms such as significant abdominal pain (especially if severe or radiating to the back), persistent vomiting, blood in stools or black stools, unintentional weight loss, or signs of dehydration