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Date of last update: 10/17/2017.

Forum Name: Endocrinology Topics

Question: Help with understanding ADH blood test

 LynnMarie - Fri Jul 23, 2010 2:53 am


I recently had my blood ADH level and plasma osmolality tested. The results were a pretty low ADH level (.8 in range of 0.0 - 4.7) but normal osmolality (283 in range of 275-295). Does this suggest that I'm just drinking too much water or are there any conditions with low ADH and normal osmolality? Thank you!
 Dr.M.Aroon kamath - Fri Jul 23, 2010 6:48 am

User avatar Hi,
Osmolality (osmolarity) shows(in lay terms) how much concentrated or dilute your plasma is. It depends chiefly upon the concentration of certain electrolytes such as sodium(Na) and potassium (K), certain other molecules such as blood sugar and urea. An increase or decrease in the water part of the plasma will thus alter the osmolality.

In health, osmolality of plasma keeps changing and mostly regulates itself. For example, when you sweat too much, plasma becomes concentrated (osmolality increases) and this acts on osmoreceptors in the hypothalamus of the brain to stimulate production of antidiuretic hormone (ADH) to be transported to the posterior pituitary gland from where it is secreted into the circulation. ADH acts
on the kidneys to withhold excretion of water so that, the urine becomes concentrated (urine osmolality increases) and plasma becomes diluted(osmolality decreases to normal levels).

in healthy adults who have no fluid restrictions and a normal activity level, normal plasma concentration of ADH is 0.35-1.94 ng/L (0.32-1.80 pmol/L). However, such low levels cannot be measured with certainty by most assays. Therefore, for best interpretation of results, ADH values should be correlated with plasma osmolality, electrolye levels(chiefly Na) and urine osmolality.

Diabetes insipidus(DI) may be caused by
1. a defect in Anti-diuretic hormone production, or
2. a defect in the kidneys' response to ADH.
Usually the patients with Diabetes insipidus pass >3litres/24 hours of low osmolality (<300 mOsmol/kg) urine.
(Defects in Anti-diuretic hormone production may arise from conditions affecting the hypothalamus/posterior pitutary (cranial,central DI or neurogenic DI).
Defect in the kidneys' response to ADH is called as Nephrogenic DI).

In the presence of nephrogenic DI, plasma ADH levels are usually elevated in relation to plasma osmolality. Conversely, with neurogenic DI, ADH levels decrease as plasma osmolality increases. In neurogenic DI, usually the plasma osmolality >295 mmol/L and serum sodium >145 meq/L and Urine osmolality is < 300 mmol/L.

ADH assays: are not routinely performed because of their complexity and lack of specificity/sensitivity. ADH assays(if available) can only help to support the diagnosis of DI along with other values as already alluded to.

Lab values you have mentioned appear to be within normal range but, as explained earlier not very helpful on their own.
Best wishes!
 LynnMarie - Fri Jul 23, 2010 5:56 pm

Wow, that was a great answer, and I feel I understand this very well now. Thank you!!
 Dr.M.Aroon kamath - Mon Jul 26, 2010 11:11 am

User avatar Hi,
Thank you for keeping in touch. It is indeed my pleasure and i am glad that it was useful. Good day!

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