A couple of things come to mind here: The parathryoid glands are usually considered to be part of the thryoid gland, not just 'close by'. It's a separate function of a component of the gland, rather than a truly separate gland. Also in hypothyroidism parathryoid hormone is increased, causing greater conversion of 0,25 vitamin D (considered semi-activated) to 1,25 vitamin D (fully active). In answer to Aidans question, the setting of limits has more to do with 'whats normal' than whats healthy. With a very highly aging population, most of the limits in the the UK are too low (in my personal opinion). The UK definition of hypothyroidism requires you to be far more ill than the dutch equivalent. So far as Vitamin D is concerned, you should take medical advice before supplementing Vitamin D is you suffer from Hyperparathyroidism and would need 1,25 Vit D checked instead of the usual 0,25 Vit D.