Hair loss for 18 months in crown area, first due to stress, then perhaps due to very low TSH as a result of Thyroxine dose increase. Iron levels fine. Hair is now finer / softer and continues to shed - will my hair ever go back to how it was?

Wrong assumptions. Iron level not relevant. "Stress" probably not a factor. Very low TSH because of overdosed thyroid probably not part of it. Thin hair on top is very common as women get older. Other meds? Birth control? Menstrual history? Pregnancies? Weight change? When you see a dermatologist be prepared to detail these items. You might be helped by spironolactone or dutasteride.