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.
I hope this helps answer your question.  If it does, please take a moment to up vote this answer on the “respond” button on the top right.  

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.