Hypothyroidism

Me, three, experientially. My (severe, but properly medicated) hypothyroidism didn't prevent weight loss, and a reasonably active exercise life didn't cause complications.

I do think all HIIT all the time is a sub-ideal plan, but not because of cortisol. It's not how elite athletes train, and they have the best possible advice from highly qualified trainers and dietitians about fitness, performance, and health. Why would us regular duffers do differently than they do?

For sure, HIIT is trendy and over-hyped now. It can play a small part in overall solid training program. But it also increases injury risk, can be likely to trigger over-fatigue and under-recovery, and more. Over-fatigue and under-recovery can hinder strength/ performance improvement plus bleed calorie burn out of daily life activity (because we drag through the day and rest more, basically).

Has it been established through repeated blood tests that you actually have persistent high cortisol, or is this just based on what you read in some article somewhere?

IMU, it is true that someone with one autoimmune condition is somewhat more likely to have other autoimmune conditions. Some of those, like Cushing's syndrome, can cause persistent high cortisol.

There are other health conditions that can cause persistent high cortisol, but that's to be sorted out by seeing medical professionals and having blood tests, not by reading articles. (Extra true if the article happens to be on a site marketing supplement/diet/exercise program "solutions", or the site of a fringe-y advocacy organization.)

Some research suggests that people with Hashi's may tend to have elevated cortisol, but I'm not sure that's been definitively proven, or is necessarily universal among those with Hashi's.

IMU, all of the "persistent high cortisol" health conditions are fairly rare, so low odds you have them, but not impossible.

Those are questions for doctors, ideally endocrinologists, not random idiots like me on the internet.

Exercise - like other forms of stress, even useful stress - tends to raise cortisol. In a generally healthy person (including those of us who are hypothyroid & properly medicated), when that person follows a sensible training plan with adequate recovery for their fitness level, that cortisol increase is temporary (maybe 30-60 minutes for something like HIIT).

Stress of all types is cumulative. If you're in a calorie deficit, have a stressful daily life, exercise (especially to excess with under recovery), aren't getting ideal nutrition, etc., those things can keep cortisol elevated a little higher or longer, maybe.

In common cases, the reasons for gradual weight gain are some combination of:

* lower daily life activity levels as we age (which can be subtle but surprisingly high impact),
* reduced muscle mass (if we don't challenge strength routinely, muscle mass tends to decrease over time), and
* eating too much (in calorie terms, which can also be subtle, especially if not tracking fairly carefully).

You don't give us many details, which makes it hard to give you specific suggestions. You haven't said how much weight gain over what time period, what specific type or how much HIIT (frequency and duration) in that time period, how many calories you're eating (and how you know), what your job and other daily chores are, how old you are, what weight/height, etc.

In theory, eating a mere 100 calories on average daily more than we burn will trigger 10 pounds of weight gain in a year. That's like a tablespoon of mayo or equivalent daily. (It can also be from a rich, somewhat indulgent meal one a week or so.)

Same effect from averaging 100 calories less movement (exercise or normal life stuff) daily, or some equivalent combination of eating a bit more plus moving a bit less. This stuff can be surprisingly subtle.

Just as context, I'm severely hypothyroid (properly medicated), female, now age 69, menopausal (of course), 5'5", lower 130s pounds, fair mucks mass for my demographic exercising pretty vigorously 4-6 days most weeks, near sedentary outside of intentional exercise. I lost around 50 pounds here at age 59-60 after previous decades of creeping overweight/obesity, the last dozen years of which I was already athletically active, even competing (not always unsuccessfully in age group comps). I got coaching certifications in my sport, so I know a bit about exercise plans and fitness development from that, plus personal study and experience.

I've maintained my weight in a healthy range since that loss. Despite being hypothroid, I require hundreds of calories more daily than MFP or my good brand/model fitness tracker estimate. That's rare in anyone, but it can happen even for us hypos. IIRC, @cmriverside above also requires more calories than estimated. Not sure where @spiriteagle99 falls on that spectrum.

If you think you have persistently high cortisol, please see a doctor, ideally an endocrinologist, for testing and diagnosis. Some of the rare conditions that can cause it are serious.

Otherwise (or simultaneously):

* make and follow a good exercise plan suitable for your fitness level (if you enjoy exercise),
* boost your daily life activity within reason,
* manage your total stress level to the extent possible,
* get a calorie estimate for moderate weight loss for your current size and activity level (including exercise either by logging exercise or by averaging it into activity level),
* shoot for decent nutrition if you aren't already,
* stay close to that calorie level on average for 4-6 weeks (say +/- 50 calories), or for least one full menstrual cycle if you have those.

After that time period, your actual real-world results, averaged over the whole time period, will give you better insight about whether and how much to adjust calorie intake to get the effect you want. For weight loss alone and per se, medical attention is only necessary if you're calorie needs seem to be dramatically less than a good calculator (like MFP) estimates.

Best wishes!