Metformin lowers TSH cureus.com/articles/50564-e...
Any link with A breakthrough-like effect of metformin reduces peripheral resistance to triiodothyronine in euthyroid, non-insulin-resistant, type 2 diabetic patients in: Endocrine Connections Volume 10 Issue 7 (2021)
Helvella?
Research suggests Metaformin also lowers B12 and folate independentnurse.co.uk/clin...
Also testosterone
This was in the Methods:
Diabetics with comorbidities, newly diagnosed and untreated hypothyroid patients and hyperthyroid patients previously treated with either radioactive Iodine or surgery and now on replacement levothyroxine therapy, were excluded from the study.
You didn’t tag helvella so I’ve just done so for you. 
The conclusion starts:
The effect of metformin in significantly lowering TSH levels in hypothyroid patients on levothyroxine therapy without causing any changes in the circulating hormone levels has raised questions regarding the complexity of monitoring such patients.
Which, to me, indicates yet another reason to look at Free T4 and Free T3. Yet they appear to prefer the clinician to wait 6 to 12 months and then look at FT4 in conjunction with TSH.
Maybe we should ask what would happen to someone who actually does need more levothyroxine? Looks like they will have to wait up to a year. Let alone if they need T3...
Whereas I see it as yet another factor undermining TSH-only testing regimes.
Anyone who takes metformin:
Long-term metformin therapy and vitamin B12 deficiency: An association to bear in mind
Infante M, Leoni M, Caprio M, Fabbri A. Long-term metformin therapy and vitamin B12 deficiency: An association to bear in mind. World J Diabetes. 2021;12(7):916-931. doi:10.4239/wjd.v12.i7.916
Close relationship of thyroid dysfunction and diabetes
Metformin Reverses Hashimoto’s Thyroiditis by Regulating Key Immune Events
Metformin
The hypoglycemic side of hypothyroidism
