While diet plays the predominant role in disorders like IBD, IBS, Celiac, NCGS, Crohn’s, and even Hashimoto’s, avoiding the trigger food doesn’t necessarily restore the patient to wellness every time. The body is incredibly resilient, but sometimes we do enough damage that it needs a hand to get to full homeostasis.
Specialty diets are the start and end, but a "middle" might be needed.
Think in stages: damage control, repair, strengthen, maintain. Before getting to maintenance mode, supplements like thiamine can make all the difference and literally change the person’s quality of life. Imagine what it’s like to wake up in a state of exhaustion every day. You haven’t so much as lifted your head from the pillow, and you’re already tired. Now imagine a natural supplement bringing energy back and chasing away chronic fatigue, without the side effects that drug treatments have.

It’s not often that we find clinical studies looking for symptom relief from food or natural supplements instead of synthetic drugs. So to see thiamine used as treatment for fatigue several times, as well as for neuro conditions, it has earned its way onto the supplement all-stars list. Thiamine (B1) is an essential vitamin that the body doesn’t make, it must come from food or supplements.
Ways to use thiamine to relieve fatigue:
1) Hashimoto’s,an autoimmune disorder affecting 15 million Americans, also called Hashimoto’s Thyroiditis (creating hypothyroidism), causes fatigue. Supplementing with thiamine can relieve Hashimoto sufferers from fatigue (1) in as little as a few days. (Chronic Fatigue is very common with Celiac as well)

2) IBD(Inflammatory Bowel Disease) (2)

3) Fibromyalgia (3)

4) MS (Multiple Sclerosis) (4)

The interesting part of that last study with MS patients is their blood tests showed normal levels of thiamine before starting the experiment. The study was conducted to see if high doses of thiamine improved their states of fatigue, and it did. This is quite significant because it begs the question: How often does a test show normal levels, yet the person might benefit and get results from clinical doses of a particular vitamin or mineral?
Thiamine and Parkinson's
Treating fatigue, particularly with disorders like Hashimoto’s where thyroid is involved is one thing, but as with all natural foods and supplements, there are always more benefits…

"…high-dose thiamine was effective in reversing PD [Parkinson’s Disease] motor and nonmotor symptoms. The clinical improvement was stable over time in all the patients. From our clinical evidence, we hypothesize that a dysfunction of thiamine-dependent metabolic processes could cause selective neural damage in the centers typically affected by this disease and might be a fundamental molecular event provoking neurodegeneration. Thiamine could have both restorative and neuroprotective action in PD."(5) 
The Bottom Line
The limited knowledge we have about nutrition keeps confirming one thing over and over: The human body was meant to run on powerful nutrition, and when we fall off the wagon, it’s powerful nutrition that usually gets us back on track.

References

(1) Costantini A, Pala MI. Thiamine and Hashimoto's thyroiditis: a report of three cases.J Altern Complement Med.2014 Mar;20(3):208-11. doi: 10.1089/acm.2012.0612. Epub 2013 Sep 25. PubMed PMID: 24351023.

(2) Costantini A, Pala MI. Thiamine and fatigue in inflammatory bowel diseases: an open-label pilot study.J Altern Complement Med.2013 Aug;19(8):704-8. doi 10.1089/acm.2011.0840. Epub 2013 Feb 4. PubMed PMID: 23379830.

(3) Costantini A, Pala MI, Tundo S, Matteucci P. High-dose thiamine improves the symptoms of fibromyalgia.BMJ Case Rep.2013 May 20;2013. pii: bcr2013009019. doi: 10.1136/bcr-2013-009019. PubMed PMID: 23696141; PubMed Central PMCID: PMC3669831.

(4) Costantini A, Nappo A, Pala MI, Zappone A. High dose thiamine improves fatigue in multiple sclerosis.BMJ Case Rep.2013 Jul 16;2013. pii: bcr2013009144. doi: 10.1136/bcr-2013-009144. PubMed PMID: 23861280; PubMed Central PMCID: PMC3736110.

(5) Costantini A, Pala MI, Grossi E, Mondonico S, Cardelli LE, Jenner C, Proietti S, Colangeli M, Fancellu R. Long-Term Treatment with High-Dose Thiamine in Parkinson Disease: An Open-Label Pilot Study.J Altern Complement Med.2015 Dec;21(12):740-7. doi: 10.1089/acm.2014.0353. Epub 2015 Oct 27. PubMed PMID: 26505466.