[annie123 (OP)]

If a hyperactive thyroid gland is treated with radioactivity and the individual has to take thyroid pills for ever after, can the thyroid gland grow again? if not, why does the dose of the medication vary throughout a lifespan? Why can 125mcg be OK for decades and then 100 for a few years and then 75 if the gland is inactive?

[alancalverd]

Total ablation of the thyroid is not always required or achieved. It is entirely normal to review all "lifetime" medications from time to time to better approximate normality.

[Bored chemist]

Other factors may change as the body ages.
The gut may get better or worse at absorbing teh medication. similarly the liver etc may get better or worse at degrading ie.
And the response of the tissues to the hormone may also change.
Any of those factors might lead to a change in the "best" dose/

[annie123 (OP)]

although 'better' is not a word usually associated with aging especially in the upper decades.

[alancalverd]

The rock musician's motto: "the older we get, the better we were."

[evan_au]

If our body mass reduces as we age, then less medication is required to reach the desired concentration (as micrograms per kilogram, or similar measures).