Another study that came up on my PubMed alerts that I couldn't find yet on here.
Obviously vitamin deficiencies are something that people here know is important to check for and a lot of people take extra B12 or generally more of other B vitamins or B vitamin complex, but I don't think I've seen anything about B1/thiamine in particular so this was an interesting thing to spot though it's only a 3 case study, very small.
EDIT: Realised I forgot to include the link to the study.
ncbi.nlm.nih.gov/pubmed/243...
"Abstract
Abstract Objectives: In a previous study on fatigue and related disorders in inflammatory bowel disease (IBD), we observed that IBD patients improved after treatment with high-dose thiamine. We hypothesized that the chronic fatigue accompanying inflammatory and autoimmune diseases is the clinical manifestation of a mild thiamine deficiency that is probably due to a dysfunction of the intracellular transport or to enzymatic abnormalities. Hashimoto's thyroiditis is both a common automimmune disease and cause of hypothyroidism. Although levothyroxine, a thyroid hormone, is the treatment of choice for hypothyroidism, a significant number of patients on thyroid hormone replacement therapy report not feeling well despite having thyroid function tests within the healthy range. Based on our hypothesis, we started treating the fatigue in patients affected by Hashimoto's thyroiditis and taking a thyroid hormone with thiamine. This is a report of the outcomes of three cases in which the fatigue component reported by patients with Hashimoto's thyroiditis was treated with thiamine. Design: Three patients on thyroid hormone replacement because of Hashimoto's thyroiditis were treated for the fatigue component of the disease from May to July 2011. Fatigue was measured using the Fatigue Severity Scale. Free thiamine in the serum and thiamine pyrophosphate in red cells were tested before and after the therapy. All three patients received oral (600 mg/day) or parenteral (100 mg/ml every four days) doses of thiamine. Results: Treatment with thiamine led to partial or complete regression of the fatigue within a few hours or days. Conclusion: As the administration of thiamine led to a partial or complete regression of the fatigue and related disorders, it is reasonable to infer that the administration of large quantities of thiamine restores thiamine-dependent processes. The mild thiamine deficiency suggested by fatigue and related disorders may be due a dysfunction of the intracellular transport of thiamine or to enzymatic abnormalities most likely related to the autoimmune process of the disease."
I did check recommended daily doses as suggest by US guidelines mentioned on MayoClinic
mayoclinic.com/health/vitam...
They said
" (RDA) for adults aged 19 years and older is 1.2 milligrams daily for males and 1.1 milligrams daily for females, taken by mouth. The RDA for pregnant or breastfeeding women of any age is 1.4 milligrams daily, taken by mouth. As a dietary supplement in adults, 1-2 milligrams of thiamine has been used daily, by mouth. "
Therefore the above mentioned doses from the study are considerably more per day than the RDA.
Though it does also say "For dysmenorrhea, 100 milligrams of thiamine has been used daily by mouth for three months. " which is the lower of the amounts used in the study so possibly that is safe to take short term? I've read it is water soluble and so unlikely to cause overdose when taken orally but no clue if what I'd read with a quick google on it is decent advice.
But the NHS website seems to to suggest taking 100mg, the lower dose in this study if I'm not getting confused there, might be okay .
nhs.uk/Conditions/vitamins-...
"What happens if I take too much thiamin?
There is not enough evidence to know what the effects might be of taking high doses of thiamin supplements each day.
What does the Department of Health advise?
You should be able to get all the thiamin you need by eating a varied and balanced diet. If you take supplements, do not take too much because this might be harmful.
Taking 100mg or less a day of thiamin supplements is unlikely to cause any harm."
Probably sensible to be cautious about it given how much over RDA those doses would be and best to see about getting tested for deficiency first if you can (though I wonder if the issues they suggest here specific to Hashimotos patients would show up on the tests? It's not clear to me) and/or consult your GP/nutrionalist about taking the supplement if you wanted to see if it helps your fatigue. Still, potentially promising.
Also, I did a little more research into what I could find linking thiamine deficiency and Hashimoto's and found:
hormonesmatter.com/post-gar...
What got my attention there was mention of yeast containing "an enzyme called thiaminase, whose action destroys thiamine" which made me wonder - and complete speculation on my behalf here - if people who swear they feel better cutting out gluten, but haven't tested positive for coeliac disease, are feeling better due to decrease in thiaminase and resultant increase in thiamine? Maybe B1 is more important and/or more problematic to digest/use for Hashimoto's patients than is realised? Food for thought there.
Hashimoto's Thyroiditis: Lifestyle Interventions for Finding and Treating the Root Cause 
