When “therefore,” “accordingly,” “nevertheless,” “consequently,” “thus,” “next,” and other such conjunctive adverbs begin a sentence, they are followed by a comma. Sometimes when they are in the middle of a sentence, they are functioning more as plain adverbs, to add emphasis, and you may or may not use commas around them. Omitting the commas is appropriate with the short adverbs: “thus,” “then,” “first,” “next.” When a longer pause in the sentence is desired, use a two (or more) syllable adverb (e.g., “therefore”) and use commas around it. When only a small break is desired, use the short form (e.g., “thus”) and no commas.
Examples: Accordingly, the profits rose.
Thus, the returns are abnormally skewed.
The returns, therefore, are abnormally skewed.
The returns are thus abnormally skewed.
The product has a peculiar sales pattern; thus, the returns are abnormally skewed.