Not all words in th- are necessarily from (Old) Norse. English tends to have th where German has d; cf. thing ~ Ding, think ~ denken, thirst ~ Dürst, etc. One important word, the third person plural pronoun is from Norse / Danish, they replaced the earlier hie. I think the OED is suggested that the alternate spelling thowel for the earlier thole was influenced by the spelling dowel. But it may be connected, especially if dowel is from Dutch or Low German where the th ~ d correspondence holds, too. Thole's a great word non-the-less.