Learn Dutch: Zij heeft hoofdpijn door de hitte. - She has a headache because of the heat.

In Dutch, medical conditions and aches like hoofdpijn (headache), buikpijn (stomachache), or koorts (fever) are typically treated as uncountable nouns. While English uses the article "a" (a headache), Dutch usually just drops een.

Could I use omdat or want instead of door to say "because of"?

Not without changing the rest of the sentence. Door acts as a preposition here meaning "because of" or "due to", so it can be followed simply by a noun (de hitte). Omdat and want are conjunctions that require a full clause with a subject and a verb. To use them, you would have to rephrase: Zij heeft hoofdpijn, omdat het heet is.

How could I express this same cause-and-effect using namelijk?

Since this lesson focuses on namelijk, you could split this idea into two sentences to use it. The adverb namelijk provides a reason for a previous statement. You could say: Zij heeft hoofdpijn. Het is namelijk erg heet. (She has a headache. It is very hot, you see). Notice how namelijk sits in the middle field of the second sentence to explain the first.

Why use hitte instead of warmte?