We have, it seems, a hypothetical imperative to brush our teeth:
(1) Brush your teeth if you want them to be healthy.
Suppose I do want my teeth to be healthy. What normative status does (1) have?
First, I could suppose that (1) has no relevant (the reason for "relevant" is that every fact are normative) normative force at all. It is simply a statement that:
(1a) Brushing teeth conduces to the health of teeth.
On interpretation (1a), a hypothetical imperative isn't an imperative at all--there is no 'ought', just an 'is'. However, it seems we get an imperative once we combine (1a) with the desire to have my teeth by healthy. But it makes no logical sense to "combine a statement with a desire" to get an imperative. That I have a desire is not a relevantly normative fact about me--it's just an 'is'. Rather, on this interpretation, we are implicitly presupposing an imperative like:
(2) Fulfill your desires (ceteris paribus).
(Or maybe more plausibly, we should work in terms of goals rather than desires--exactly the same points apply.) And this imperative is not hypothetical in the relevant sense (it's conditional in respect of the "ceteris paribus", but that's not the relevant sense of "hypothetical")--its force cannot be dependent on what our desires are, at pain of vicious circularity. So, on this interpretation, hypothetical imperatives are not really imperatives, but we get imperatives when we combine them with some imperative like (2).
The second option is that (1) is genuinely relevantly normative: it expresses a genuine imperative. The imperative is conditional, but in that respect it does not differ from:
(3) If you have promised to whistle Yankee Doodle, you should whistle Yankee Doodle (ceteris paribus).
I do not think that when people are talking of "hypothetical imperatives" they simply mean "conditional imperatives". That isn't the relevant sense of "hypothetical". The duty to keep promises if you've made any is conditional, but not relevantly hypothetical--it applies no matter whether you desire it to or not.
Now the folks who say that morality is a system of hypothetical imperatives do not simply mean that all moral truths are conditional in form. Rather, they mean that the imperative force of morality comes from us adopting morality as a goal or having a desire to live morally. But all this once again presupposes something like (2), of which the imperatives not to murder if you want to be moral and to brush your teeth if you want to be healthy are consequences.
So, it seems to me that hypothetical imperatives presuppose categorical (in the relevant sense) ones like (2). And (2) is not all that plausible, I think. But suppose (2) is true. Then, we can ask what the status of (2) is. Why should I follow my goals? Why should I do what I want? I suspect that any plausible justification of (2) will be broadly moral in nature, based on some notion of human desires as reflective of the good, or of humans as having a duty to be true to themselves. Otherwise, it is not plausible that (2) should have any real authority. (And of course it would not do to ground the authority of (2) in terms of a higher order desire to follow my desires.) If so, then the notion of morality as a system of hypothetical imperatives has imploded.