@isabelle7,
the endocrinologist I see must think it will keep working for me, because he said that I can stay on it until my T-score is -1. I don't think we ever get back to -1. So I think he is saying (without using the word my cardiologist is fond of) until I finish my life.
It has been noted that usually the cells that resorb bone become more active than the cells that break bone down after about 18 months on Forteo or Tymlos. This process can be charted with bone markers.
Isabelle, I haven't tortured anyone else with my theory about this. You'll be the first and remember that I don't have a medical background.
Forteo is only the first 1-34 amino acids of the parathyroid hormone. We take it and bone turnover ensues at a good pace. It is the last sequence 55 to 84 that cause the cells that break down bone to self destruct. Soon after taking Forteo our parathyroid glands reduce production, measured by serum pth. It only takes a month off Forteo for the parathyroid gland resume producing the 55-84 acids and reduce the number of cells breaking down our bones.
They say response the second round of Forteo is as robust as the first. And your husband will be in the perfect position at 4 or 5 week after Forteo to experience that robust response.