Is Seventh Day Adventistism considered an orthodox denomination?

The answer is, yes. Seventh-day Adventism embodies the true, original, historic, ancient, Eastern Orthodox Church. Their views line up with the eastern churches of Syria from the 2nd, 3rd, 4th and 5th centuries. That's right. Yes they do.

I'll first direct you to a decent summary of SDA differences from typical (i.e. common) protestants: What are the main differences between 7th Day Adventists and Catholic/Protestant churches?

Now let's examine a few definitions of orthodox.

orthodox adjective
  or·​tho·​dox | \ˈȯr-thə-ˌdäks \

    Definition of orthodox (Entry 1 of 2)

      1a : conforming to established doctrine especially in religion
        // orthodox principles
        // the orthodox interpretation
      1b : CONVENTIONAL
        // took an orthodox approach to the problem
        // orthodox medicine

      2 capitalized : of, relating to, or constituting any
      of various conservative religious or political groups: such as
        a : EASTERN ORTHODOX
          // Greek Orthodox rituals
        b : of or relating to Orthodox Judaism
          // Orthodox Jews in their black suits and hats and modest dresses
- Merriam-Webster

In a religious academic setting, Orthodox is almost always used as in the second definition. That is, it is a specific thing, a proper noun. In this case, Seventh Day Adventist is trivially not Orthodox.

Outside of the academic setting, but still within highly religious settings (i.e. casual Bible studies, sermons, religious books for popular consumption) orthodox takes more of the first definition. The scope of it's use tends to coincide with many preferring to call their own beliefs "orthodox", which is to say they believe what they think is right and all biblically significant characters believed it as well. In a phrase, they think what they believe is historically correct. Many protestant denominations believe this about themselves.

However, the fact is most of them do not fit this definition. They have little to no evidence that their particular beliefs were common or even existed before the founding of their denomination. What is clear is that Catholics have quite a broad leg to stand on in this regard, and they are not shy to mention it as a point of validity for themselves. And in a broad sense, "Catholic" refers also to those ancient denominations that take the name "Orthodox". These are the first denominations and they have the history to prove it. Therefore, even in these settings, "orthodox" should be reserved for only these old denominations because they have the traditions of the first Christians.

As Protestants are outsiders to Orthodoxy, Seventh Day Adventists are outsiders to Protestants. It is clear that they have quite a few beliefs that most Protestants roundly reject and would even call heresy. And SDAs gladly accept this image. They understand they have some starkly different beliefs on some really big topics, and it is on exactly those topics that they proselytize. They preach consistently on keeping the Sabbath and the mercy of Annihilationism, neither of which are tenets of common protestant denominations.

The answer is clear: No, Seventh Day Adventists are not orthodox. They do not use the label for themselves and no other denomination calls them such. The word simply doesn't fit with any definition.

I came across my own answer here a year later and notice one missing point that is probably related. Adventists consider themselves a "remnant church". They believe strongly that their beliefs are the original beliefs of Christianity and that remnants of their orthodoxy has survived through the ages (claims of African churches mostly), though they do recognize that their prophet, Ellen White, is responsible for their creation as an American denomination. Contrast this with LDS (mormons) who believe that they are a restoration of a once lost orthodoxy.

A caveat: As you tighten scope of any labeling effort, you will often find the word "orthodox" used in conjunction with other specific terms. This is usually to denote that within a denomination there are disagreements and even schisms. For example, see the Anglican Orthodox Church and the Orthodox Presbyterian Church. Both already protestant and distinctly not Orthodox Anglican and Presbyterian Churches split into one or more denominations, with one keeping closer to their respective traditional beliefs. The SDA is no stranger to this usage of the word, as this article references an "Adventist Orthodoxy". Given some time and a significant disagreement, we may see the birth of The Adventist Orthodox Church, but that name would only imply that it holds to the traditional beliefs of Adventists.