It's extremely common among thyroid patients, especially after a hyper metabolic experience. Other things that can contribute are eating sugar, drinking tea, coffee and alcohol, eating raw fish, and certain medications.
"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.
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.