Hyponatremia is a low sodium concentration in the blood. It is generally defined as a sodium concentration of less than 135 mmol/L (135 mEq/L), with severe hyponatremia being below 120 mEq/L. Symptoms can be absent, mild or severe. Mild symptoms include a decreased ability to think, headaches, nausea, and poor balance. Severe symptoms include confusion, seizures, and coma.The causes of hyponatremia are typically classified by a persons body fluid status into low volume, normal volume, or high volume. Low volume hyponatremia can occur from diarrhea, vomiting, diuretics, and sweating. Normal volume hyponatremia is divided into cases with dilute urine and concentrated urine. Cases in which the urine is dilute include adrenal insufficiency, hypothyroidism, and drinking too much water or too much beer. Cases in which the urine is concentrated include syndrome of inappropriate antidiuretic hormone secretion (SIADH). High volume hyponatremia can occur from heart failure, liver failure, and kidney failure. Conditions that can lead to falsely low sodium measurements include high blood protein levels such as in multiple myeloma, high blood fat levels, and high blood sugar.Treatment is based on the underlying cause. Correcting hyponatremia too quickly can lead to complications. Rapid partial correction with 3% normal saline is only recommended in those with significant symptoms and occasionally those in whom the condition was of rapid onset. Low volume hyponatremia is typically treated with intravenous normal saline.
Secondary causes are far more common than primary ones. One study found that 15% of healthy first-time mothers had low milk supply 2–3 weeks after birth, with secondary causes accounting for at least two-thirds of those cases.Breastfeeding management issues that can interfere with regular milk drainage from the breast include poor latch, unnecessary use of supplemental formula, timed or scheduled feedings (as opposed to on-demand feedings), and overuse of pacifiers. Medical conditions in the infant that result in weak or unco-ordinated sucking can cause low milk supply by inhibiting the transfer of milk to the baby. These conditions include tongue-tie, congenital heart defects,  prematurity, and Down syndrome.Primary causes of low milk supply include:

insufficient glandular tissue, also known as primary lactation failure. Breasts with insufficient glandular tissue often have a tubular shape, with a large space between the breasts, and often do not grow during pregnancy. retained placenta
Sheehans syndrome
prior breast surgery (especially breast reduction)
prior nipple piercing
polycystic ovary syndrome (PCOS)
hypothyroidism
hypoprolactinemia
theca lutein cysts
hypertension
pregnancySmoking more than 15 cigarettes per day is associated with reduced milk production. Many medications are known to significantly suppress milk production, including pseudoephedrine, diuretics, and contraceptives that contain estrogen. It is suspected that some herbs, particularly sage and peppermint, suppress milk production. Mechanism
The physiological mechanisms that regulate breast milk supply are poorly understood. High levels of prolactin are necessary for lactation, however there is no direct correlation between baseline levels of prolactin and quantity of milk production.