Hi
@Lady_with_3_dogs and welcome to the forum
To be honest, I don't know the answer to your question, particularly as there are loads of possibilities.
Metformin, along with PPIs and antacids, are known to affect the absorption of Vit B 12 in many people. Also, if you have Hypo, or Hyperthyroid issues, absorption could be compromised.
However, I've recently been looking into Vit B12 deficiency and remembered that loss of smell is mentioned. The reason I suggest asking your GP to check your Vit B12 levels (along with Folate, Ferritin and Vit D) is that, assuming you have been taking Metformin, you may have developed an absorption issue.
Here are some links to get you started
Scroll down to 2nd category – Neuropsychiatric – at bottom of list
http://www.b12deficiency.info/signs-and-symptoms/
Loss of Smell & Vitamin Deficiency
http://www.livestrong.com/article/517337-loss-of-smell-vitamin-deficiency/
NHS Choices – Loss of Smell
http://www.nhs.uk/conditions/anosmia/Pages/Introduction.aspx
ThyroidUK and Vitamins
http://www.thyroiduk.org.uk/tuk/treatment/vitamins.html
However, please don't think you might have found an easy answer as many GPs and Specialists seem to believe such issues are unlikely to be worthy of treatment and that 'normal' levels means 'no problem'!!! It appears the ranges used are not particularly accurate and the medics are ill-informed, on the whole.
Good luck. I'm sure many knowledgeable members will have further advice and suggestions