My heart rate was crazy when my thyroid problem was out of whack. Sometimes my resting pulse was around 100. And it would take hours after a bike ride to dip below 100.
But now, about a year after it's more or less under control, my resting HR is around 65. It'll hit 160+ during vigorous bike rides but usually drops pretty quickly, although it still tends to stay over 80 for a couple of hours.
Also depends on whether I needed Sudafed for sinus congestion. That tends to elevate my resting and active HR about 10 bpm, although it doesn't seem to affect my maximum heart rate.
And I've been using an HRV app to monitor it as part of my usual fitness routine. Helps notify me when I need to rest and when I'm good to push harder. So far, after several weeks of near-daily use, it's been remarkably accurate.
Interval training has helped too. After about a 30 minute warmup I can see during rides that my HR elevates appropriately with effort, and quickly drops when appropriate. Still takes me a long time to warm up so the HR stabilizes, but my cooldown period is less critical than it was.