I'm 5'5" and weigh 120 lbs. I'm a 34DD and would love to be at most a 34C. Is this something insurance would cover? (Photo)

It is likely that your would qualify for a reduction but the final determination is based on an estimated amount of reduction by your plastic surgeon, your insurance plans criteria, your symptoms and your height and weight. In general, approval from an insurance company for a breast reduction involves meeting several criteria. Each insurer will have a minimum resection volume (weight) of breast tissue that they consider constitutes a medically necessary reduction, often in the range of 400- 500 grams. An experienced Plastic Surgeon can estimate the expected volume of resection. Some insurers use the Schnur Sliding Scale that is used to determine an appropriate cut-off for breast reduction resection weights based on the patient's height and weight. Some will use Body Mass Index as an exclusion criteria if the patient is deemed overweight resulting in large breasts Your best bet is to contact your insurer and find out what the specific criteria are for you. In addition, in the approval process your doctor(s) should document persistent neck and shoulder pain, painful shoulder grooving from bra straps, frequent rashes of the inframammary fold, and frequent episodes of headache, backache, and nerve symptoms that are the result of large breasts. Some insurers require a period (often 6 months) of conservative management for musculoskeletal symptoms with physical therapy, although this has not been shown to be a successful treatment. In addition, physical therapy will not address many of the other symptoms including bra strap grooving, rashes, inability to function social and in athletic endeavors to name a few. Photographs are a key component of the pre-authorization process. A picture can convey the true nature of the problem that might not be clear from the objective data submittted. Unless you have other problems that might prevent you from having surgery, it looks like you should be a good candidate. The sooner you see a Board Certified Plastic Surgeon, the sooner you can get your approval process going.Good luck.

Easy to write YES but it depends upon the health insurance plan you have, restrictions, or pre qualifiers that only you can determine. Next see a boarded PS too address a pre determination l;ether to the insurance company ... 

Thank you for the question and pictures. Based on your description, you may be a good candidate for breast reduction surgery. “Official” recommendations of course would necessitate in-person consultation.
Breast reduction surgery is one of the most patient pleasing operations we perform. By removing “excess” breast tissue, adipose tissue, and breast skin  this operation reduces AND lifts the breasts to a higher position on the chest wall (in other words, the breasts are "automatically" lifted when a breast reduction is performed). By doing so, patients often find improvement in neck, back, and shoulder discomfort and find it easier to form their activities of daily living and exercise.
Timing of the operation will depend on the patient's life circumstances. Sometimes breast  reduction surgery is covered through health insurance. The best way to obtain insurance coverage for breast reduction surgery involves some “hoops” to jump through. The more documentation you have (for example, from your primary care doctor, physical therapist, chiropractor etc.) the better when it comes to obtaining insurance “authorization” for the procedure.
 This documentation and letter/pictures from your plastic surgeon will help you obtain authorization.
When the time is right,  seek consultation with well-trained/experienced board-certified plastic surgeons. Ask to see lots of examples of their work and communicate your goals clearly. Best NOT  to communicate your goals in terms of achieving a specific cup size; this type of communication can be a source of miscommunication. In my practice, I prefer the use's of goal pictures during this communication phase;  many patients wish to remove enough breast tissue to improve/alleviate symptoms while maintaining upper/lower body proportionality.

I hope this, and the attached link (dedicated to breast reduction surgery concerns), helps. Best wishes.