Thiol S-methyltransferase that catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to alkyl and phenolic thiol- containing acceptor substrates. Together with TMT1B accounts for most of S-thiol methylation activity in the endoplasmic reticulum of hepatocytes (PubMed:37137720). Able to methylate the N6 position of adenosine residues in long non-coding RNAs (lncRNAs). May facilitate lncRNAs transfer into exosomes at the tumor-stroma interface (PubMed:34980213). Promotes osteogenic and odontogenic differentiation by regulating the expression of genes involved in stem cell differentiation and survival (PubMed:34226523, PubMed:34790668). Targeted from the endoplasmic reticulum to lipid droplets, where it recruits cellular proteins to form functional organelles (PubMed:19773358). {ECO:0000269|PubMed:19773358, ECO:0000269|PubMed:34980213, ECO:0000269|PubMed:37137720}. (Microbial infection) May be involved in the assembly and release stages of hepatitis C virus (HCV) life cycle and thus play a crucial role in HCV propagation. . This is the function of TMT1A (thiol methyltransferase 1A, Ensembl gene identifier ENSG00000185432).