The patient was a 60-year-old man who was found to have a high HbA1c level (10.0 %) during a visit to his primary care physician due to weight loss of 7 kg/year, and was then referred to our department. He showed a casual blood glucose level of 319 mg/dL and was diagnosed with type 2 diabetes, while his serum triglyceride level was ≥10000 mg/dL (reference value) and his serum sodium level was low (114 mEq/L), as measured by indirect ion selective electrodes (ISE) using an automatic biochemical analyzer. Since he was asymptomatic and his serum sodium level (as measured by a blood gas analysis [direct ISE]), was within the normal range, we concluded that severe hypertriglyceridemia caused pseudohyponatremia. With diet, glycemic control, and antihyperlipidemic drugs, the discrepancy between his serum sodium level, as measured by indirect ISE, and his serum sodium level, as measured by direct ISE, gradually decreased and eventually disappeared with a decrease in the serum triglyceride level. Although it is generally known that hypertriglyceridemia induces pseudohyponatremia, very few studies have been reported on pseudohyponatremia with serum triglyceride levels exceeding 10,000 mg/dL, which makes the present case valuable. In conditions in which the ratio of solid to water in plasma is altered (hyperlipidemia, hyperproteinemia, etc.), it is desirable to evaluate electrolytes by direct ISE.

A 17-year-old boy and his younger sister, a 15-year-old girl, were suspected of having diabetes mellitus based on a positive result according to a urinary glucose test at a school physical examination. They initially had normal findings on the oral glucose tolerance test, but they both developed diabetes mellitus within a few years of the start of the clinical course. They were genetically diagnosed with MODY3 with a c.1298C>T mutation in the hepatocyte nuclear factor-1 alpha (HNF-1α) gene and were simultaneously diagnosed with primary dyslipidemia, exhibiting type IIb or IV manifestation. Their clinical course, such as their insulin secretion ability, and the dose of insulin injection required were similar. In this family, only the members with MODY3 exhibited primary dyslipidemia, with all members without MODY3 not showing primary dyslipidemia. Interestingly, their father, who had the same mutation in the HNF-1α gene, did not develop MODY3 or primary dyslipidemia, these observations suggest that the development of MODY3 due to an HNF-1α gene mutation may be closely associated with that of primary dyslipidemia.