I'd been slowly coming back this year from a few injuries and general funk. Things were progressing steadily through the late winter, spring, and early summer. Then ... it just started going the wrong way. Despite the weather becoming more accommodating (less unbearable?), my pace rose, my HR rose, and breathing felt increasingly more labored than it should for the pace. Running the same routes, and I can't really point to anything that's changed.

So since it affected my running, I went to the doctor! Hypothyroidism runs in the family, so I asked for that to be tested. TSH came back elevated, but I don't see a T4 result. Dr. Google says that'd make it either subclinical or overt hypo.

But before I chase zebras ... anyone want to toss out some other cause?

(And yeah, that's only easy runs -- I corrected for trails, stroller runs, and anything else that artificially slowed me.)

I second the need to find an athlete-understanding doc.  With both thyroid and iron, I struggled to get good care until I found someone who understood the difference.  Two docs would not diagnose iron-related issues because they did not care until the anemia was so debilitating that a patient was suffering to function in normal daily live.  One told me point blank "if I can't see it in you when you try to stand up out of your chair, it's not concerning to me."  Clearly this is way past the point that it affects an athlete.  Low ferritin, even without more than low-normal RBC or hemaglobin values, DOES affect an athlete.  This is also an issue fitting your symptoms that you should chase down.

Similarly, most docs I've found go straight by the numbers on thyroid.  Mine was borderline.  I was medicated for a number of months, and at first it seemed to make a significant difference.  Eventually it did not.  I am convinced I still have an undiagnosed endocrine issue (my basal body temp has dropped down to 96.X or even 95.X some days) but have given up trying to get an answer.