What is the best treatment for a crepey neck? I have hypothyroidism, so it must be a treatment safe for hypothyroidism.

The neck can get saggy for many reasons including a lot of sun exposure, weight gains or loss, pregnancy etc.  Aging of the neck is very common before aging of the face.  It's one of the things that can be very frustrating to patients of all ages. 
The face and neck keep changing as you age. Some of the changes are at the skin level, some at the deeper levels like the fat, retaining ligaments or muscles. There are several nonsurgical and surgical options to treat each of these different areas. Some can be done together and it really depends how much improvement you want to see as well as what part really bothers you. If there is significant sagging, surgery is usually the preferred procedure but can still be combined with injections and laser treatments before or after surgery.  
A simple way to think about it - think of surgery as addressing any excess skin and laxity as well as removing fat.  Think of laser as tightening the skin levels. Injections like Kybella can nonsurgically help diminish the fat and offer improvement and there are all sorts of laser procedures that can tighten different layers as well.
Most of these procedures can be done safely in the office and the results are very satisfying. An in-office private consultation and examination with a board-certified plastic surgeon would be the best way to fully see and address your specific concerns and determine the right treatment plan.
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Without photos it is hard to tell you which is best for you but the 2 best options are Fractora RF radiofrequency to tighten up skin or a neck lift platysmaplasty if the skin is really saggy.

Thanks for sharing. In general, current neck rejuvenation procedures include:
A NECK LIFT* with a LOWER FACELIFT would be the gold standard and gets the best results usually reserved for patients about mid to late 40's and older. You also achieve jawline improvements.A NECK LIFT* stand alone would be the next best procedure and tightens only the neck.CHIN LIPO and/or LASER LIPOLYSIS if only fat removal is required with good skin elasticity, typically in a younger patient. A Platysmaplasty can be added for further muscle tighteningPROFOUND RF, FaceTite, and ULTHERAPY are FDA approved minimally invasive tightening technologies using using radio frequency or ultrasound energies. KYBELLA injections for a double that melts fat under the chin and takes a few sessionsBOTOX to the platysma bands to decrease their showFractional CO2 Laser skin resurfacing helps with skin texture, fine lines, and brown spots*It is important the a Neck lift involves some liposuction and importantly a platysmaplasty which is a procedure that tightens the underlying loose neck muscles. 
Without a photo the appropriate treatment can not be definitive.  What can be said is that all the above treatments have no bearing on your thyroid medical condition so they are all safe.  If one is having thyroid surgery then that should take priority.  Best to be examined in person. Talk to several board certified surgeons experienced in neck and face lifting.
 

As we age the skin sags there is nothign you can do that but try preventative treatments or get a neck/facelift earlier in the aging process.  There are numerous options and some can be used in combination to get best outcomes but you need a consultation with an expert.
Options include for skin tightening :
thermage radiofrequency to heat skin and tighten it over time and prevent the aging process
ultherapy highintensity ultrasound to contract muscle and tighten the face and neck and prevent the aging process
PDO nova threads or instalift threads for immediate lifting without surgery and long term collagen production
sculptra injections for collagen production to areas of the face and neck with crepe skin or early sagging
btl exilis ultra, venus legacy, and thermi250 for constant collagen stimulation monthly with facials
facelift/necklift with fat grafting/fillers if you need a big improvement
ThermiRF or J plasma or Inmodebodytite  internal heating and tightening to long term internal heating and tighting with or without surgery
Platysmaplasty to tighten the muscles of the neck long term and create contour
Facial implants or fillers to contour the jawline and chin to improve the apperance of the nec
Best, Dr. Emer.

As long as your condition is controlled, there should be no problem doing a necklift If indicated. It’s best to have a personal consultation to make this determination.

A little bit of excess skin and/or fat under the neck can be removed with a SIMI (single incision, minimally invasive) neck lift. Unlike a typical neck lift, which consists of 3 incisions - one under your chin and two behind the ear, this only requires one incision in the crux of your neck. From this incision, liposuction is performed to remove fat, the skin is undermined and draped to address the excess skin, some fat is excised directly and the platysma muscles are tightened. All of these steps help contour and smooth out the neck. As long as your hypothyroidism is controlled and you receive clearance from your primary care physician, this procedure is a great option to address your "creepy neck" concerns.

 First of all, it is important to be evaluated for hypothyroidism and be on medications for it. Probably best to get medical clearance from your endocrinologist before undergoing elective cosmetic surgery. A full set of facial photographs and an in-person examination going to be required to make a determination about the degree of looseness of the skin in the neck. Loose and inelastic skin in the neck is usually best treated with a lower face and neck lift. For many examples, please see the link and the video below

Thank you for the question and I would go on some complimentary consultations with experts and compare recommendations after a full examination and look at photos of similar patients and become fully informed about the desired procedure Dr Corbin

There are various non-surgical treatments available that may have application for you or the more traditional neck lift which I do under a local anesthetic..