I went through two of them. The first time was due to the rapid rise in T3 when I was on T3 only, as described above. As I said, my tendons not only hurt, but eventually felt incredibly “weak” and “floppy”. One day in Week 3 of T3 I heard and felt a huge “pop” in my left biceps tendon. Thank goodness it didn’t rupture, but pain started radiating down my hand down through some of my fingers, and back up through my shoulder. Within days, the left shoulder started “freezing up”. I lost about 85% function of my left arm as a result for several months, and it took a good 9-12 months to resolve completely. “Frozen Shoulder” is a good description of it; I’ve heard attempts to “unfreeze” or “unlock“ the shoulders to restore range of motion even under anesthesia does NOT work without causing significant damage. It’s simply a matter of waiting it out. The second shoulder, the right one, froze up when a rapid rise in T4 occurred after I started medication because I had inadvertently gotten too “low” on TH overall. At that time, I attempted to restore TH status with T4, and within several weeks, as T4 slowly built up in my system, the right arm and shoulder developed pain and “froze up”. This reaction was more delayed and was much milder overall than the left arm however, because of the longer half life of T4, and the decreased metabolic activity of T4. I estimate I lost about 20-40% of the use of my right shoulder and arm that time. The worst of symptoms lasted about a month or two, and resolved pretty quickly after that. Review “Frozen Shoulders and Thyroid” on the internet to see the association of this disorder with thyroid disorders. Without intending to, I confirmed this association for myself. What this means to me is that, if a person suddenly develops “Frozen Shoulder(s)”, one of the places to look for problems in terms of causation would be the thyroid axis and a surge of endogenous TH, most notably, T3. I would do a full thyroid panel test, with antibodies in particular, as the TSI, TrAb, and TBII antibodies are associated with Grave’s disease, which may be contributing to a thyrotoxicosis reaction, thereby causing the frozen shoulder(s). All TH antibodies, including TPO and Tg, will significantly affect TSH values secondary to T3 flaring, so using TSH as a screen will not be diagnostic of true thyroid pathology in this situation.