Which Medical Specialist, If Any, Should I Take Her To?

Uneven development is normal, most women do not have exactly same sized breasts and it is likely that the smaller breast will catch up, but never be completely even.
If you are concerned about her development you should take her to an OB/GYN first. It would be a good idea to build a rapport with one now, if she has her period she may want to start regular exams as well.
For the acne the GYN may recommend birth control pills or refer you to a dermatologist. Many people are hesitant to put their teenagers on the pill, however, if topical acne medications are not working for her (try prescription strength from dermatologist) I would consider it, as the pill as much less severe side effects than many systemic (oral) acne medications.
Good luck

I'd take her to a regular gynecologist to start with and they can recommend a specialist if needed.

I'd start with the pediatrician but be very proactive and verbal with your concerns, dont let the pediatrician blow it off as nothing unless he/she can convince you that it's so.
You have to be careful with puberty, you can't micro-manage it.... it's a phase and for most of us, everything evens out eventually.

a visit with a pediatric endocrinologist might be in order to see if there are medical conditions that need to be addressed. check with your local childrens' hospital to find one. They see a lot of teens, not just children. good luck!

I think since it is mostly female issues, you should take her to a GYN.

OB for the worries about her breasts and the family history of hypothyroidism. A dermatologist for the acne.

Don't forego the dermatologist just to save money on doctor's visits. The dermatologist can make a huge difference in the treatment of her acne.

A dermatologist for the acne - they have some prescription medications that can really help.
A friend of the family's teen boy had a HORRIBLE time with almost total body acne and with a dermatologists help he came out of it with no scars (which was amazing considering how bad he had it).
For the uneven breasts - I'm not sure there is anything anyone can do short of a boob job and they really don't want to do that until she is finished growing.
Some times these things will even out when she's grown up, married, has a child of her own and is breast feeding (a boob job might mess that up).
We have hypothyroid and hyperthyroid in the family and there are a lot of symptoms but I don't think I've ever heard of uneven breast development as one of them.
The only other thing to do is buy bras for the larger side and pad the smaller side to balance her out.

Does the pediatrician know about the family history of hypothyroidism? And if so and you are not happy with that, get a second opinion. Always go with your gut. As for the acne, Neutrogena makes a great line of products, including body wash. Try products with Salicylic Acid or benzoyl peroxide. Saicylic acid products are easier on the skin. Benzoyl Peroxide may be drying and cause allergy. My son developed an allergy to the Benzoyl Peroxide and can only use Salicylic acid. You can also try Proactive. My oldest used it and was happy with it. Get a second opinion and follow your gut.

Take her to a dermatologist and have them run a blood panel. My daughter turned out to have a severe thyroid problem. Now that it has been taken care of, her skin has really cleared up.

My sister had the same issue. My mom took her to the gyno and the gyno has been dealing with it. (Sis is 20 now and wears a special bra where the one side is padded in order to make up for the huge difference in breast size.)

The severe acne should be treated by a dermatologist. If you don't need a referral from the pediatrician, go ahead and make the appointment with the endocrinologist. Doc wants to wait til she's done growing? For girls, usually at 15, they are done. How long has she had her period? Usually growth stops 2 years later.

There is a condition called hypoplasia of the breast, which an OB/GYN could diagnose. It simply means a lack of breast tissue. I know this from personal experience (and having it does not necessarily interfere with nursing -either with or without corrective surgery).

The OB could probably analyze hormonal levels and figure out if there are any issues there. A visit to a derm may also be in order. You definitely should pursue this now, not wait, because even if there is nothing to do but in fact wait it out, there is the real chance that these conditions will begin to impact her self esteem. At least investigating now arms both you and your daughter with knowledge about what's really going on. And maintaining self esteem through a difficult puberty is priceless.

D.,

I think that even when the pediatrician told you to wait, you still may want ask her/him to give you some referrals for specialists she/he thinks your daughter should see either Ob/Gyn, or endocrinologist, etc. Tell the pediatrician, nicely, that you don't want to wait, and take your daughter to see someone who can help her or at least give you both peace of mind since you feel it is necessary to address this soon. There are many things H. that need, in my opinion, to be addressed.
I would do that in your case.