How long have you been taking 100mcg. Have you had dose reduced or were you previously on T3 as well, but now stopped?
You are significantly under medicated and need dose increase. 25mcg and retest in 6-8 weeks
Do you also have high thyroid antibodies? You need to know. Did GP or Endo ever test these? If not ask that they are tested.
If your antibodies are high this is Hashimoto's, (also known as autoimmune thyroid disease). About 90% of hypothyroidism in UK is due to Hashimoto's.
Hashimoto's very often affects the gut, leading to low stomach acid, low vitamin levels and leaky gut.
Low vitamins are common when hypo, the ones that affect thyroid are vitamin D, folate, ferritin and B12. When they are too low they stop Thyroid hormones working.
Have these been tested recently, if not ask GP to test ASAP
According to Izabella Wentz the Thyroid Pharmacist approx 5% with Hashimoto's are coeliac, but over 80% find gluten free diet helps significantly. Either due to direct gluten intolerance (no test available) or due to leaky gut and gluten causing molecular mimicry (see Amy Myers link)
But don't be surprised that GP or endo never mention gut, gluten or low vitamins. Hashimoto's is very poorly understood
Changing to a strictly gluten free diet may help reduce symptoms.
Low stomach acid can be an issue
Lots of posts on here about how to improve with Apple cider vinegar or Betaine HCL
Other things to help heal gut lining
If you can't get full thyroid and vitamin testing from GP
Medichecks Thyroid plus ultra vitamin or Blue Horizon Thyroid plus eleven are the most popular choice. DIY finger prick test or option to pay extra for private blood draw. Both companies often have money off offers. DIY finger prick test or option to pay extra for private blood draw or
All thyroid tests should be done as early as possible in morning and fasting and don't take Levo in the 24 hours prior to test, delay and take straight after. This gives highest TSH and most consistent results
You need a new endo.
Email Louise at a Thyroid UK for list of recommended thyroid specialists firstname.lastname@example.org