First thing to do is differentiate whether he is hypovolemic, euvolemic, or hypervolemic. Sounds like you thought he was euvolemic (since you diagnosed him with SIADH).

Hypervolemic hyponatremia is often easier to diagnose clinically. The challenge comes when you're not sure if the patient is mildly hypovolemic or euvolemic. In this case, your patient had GI symptoms? Diarrhea? If so, this might have made him mildly hypovolemic to account for the low sodium. One approach would be to increase fluid intake (oral or IV) and watch the response closely... hypovolemic hyponatremia should improve, while euvolemic hyponatremia (including SIADH) would actually get a little worst.

Aside from this trial-and-error approach, a neat little trick to help differentiate the two, especially for more concerning cases of hyponatremia (lower Na), is to measure urine electrolytes. If the urine Na is low (some say less than 20 as the cut off, others say less than 40), this would strongly suggest hypovolemic hyponatremia, in which case increasing oral fluid intake (or giving IV fluids in more severe cases) is the best approach, and fluid/ Na restriction would actually make things worst. The one caveat to this is that it cannot be used if the patient is on a loop diuretic (and possibly might also not be as accurate with a thiazide diuretic) because you are wasting salt in urine by the very action of those agents, irrespective of volume status. In these cases, you can measure urine urea and calculate a fractional excretion of urea to get the same information about possible hypovolemia.

I would argue based on the facts you presented, that perhaps the thiazide diuretic was the culprit (unless he had diarrhea as part of his presenting symptoms, in which case he may have been a little on the dry side and may have needed increased fluid intake). But honestly, if his lowest sodium was 133, I wouldn't have worried too much about it. I would ask the GP to repeat at some point the next week to make sure it's not any lower. No need for admission for a sodium of 133 on its own. 

Hope this helps!

He was on Thiazide Diuretics until he had SIADH.

His Na when he had SIADH (20 days ago) was 103mEq/L. 

He didn't have diarrhea when he had SIADH, nor does he have it now. But he had fleet enemas from tap water (colonic irrigation) five days in a row immediately before he developed SIADH because he was constipated and had abdominal pain.

With [Na+] 133 you do not have to worry, however, your point I guess, if he dropped his [Na+] 6 mmol/l in 9 days he might keep doing so, and 10 days later it might be 125 or less.

Before drawing any conclusion you need another important piece of information: urine osmolality and urine Na level, did you check theme when his [Na+] was 103? 

Dr. Yousef, his urine Na+ was 47mmol/L when his serum Na+ was 103mEq/L.

would we do urine specific gravity test to find a cause of hyponatremia?

Dear Lay, it was done on the same time, when Na was 133mEq/L Specific Gravity was 1.025.

What about urine osmolality?

so far I'm not convinced he have true SIDH for many reasons. at the time of the blood work he was of thyroid treatment which plays role in sodium balance at the kidney level. the man received multiple free water enemas and was on diuretics and was septic. so the picture was confounded by many factors

I would suggest resuming the thyroid meds (unless there is contraindication), evaluate fluid status clinically (vitals+weight), then repeating serum Na, serum osmo, urine Na, urine osmo, urine specific gravity. then if you are still concerned about SIDH to do pituitary work up & search for small lung cancer. However, I will strongly recommend including an endocrinologist and/or nephrologist to help out with the work up  

It was not done Dr. Boobes

Thank you Dr, Khalifah

what about the status of his adrenals. check for adrenal insufficiency.

Thank you Dr Ates

The following investigations might help in treatment of this patient - measurement of urine Na & osmolarity .  A low urine Na & high urine Osm would suggest hypovolemia as a cause of Hyponatremia . A normal urine Na & high urine Osm with euvolemic status would suggest SIADH . A similar urine Na & Osm status can also occur in hypervolemic status , such as cardiac failure .

   The fact that he had thiazide diuretics & enema suggest that he would have had hypovolemia as a cause of hyponatremia & in addition , his anti - depressant might have also contributed to hyponatremia . The improvement of hyponatremia from 103 to 139 mEq suggest that the restricted water & high salt intake has worked  His 133 mEq , at present might also suggest hypovolemia which needs evaluation . Please continue the same protocol of treatment as this has corrected his hyponatremia during his previous admission .