Vitamin B1/Thiamine test can help your doctor diagnose a deficiency of vitamin B1 or thiamine in your body. Vitamin B1/thiamine deficiency can cause muscle weakness, fatigue, and reduced reflexes. Severe deficiency can lead to Beri Beri, a disease that can affect the heart and nerves, leading to serious complications. A simple blood test can help your doctor detect vitamin B1 deficiency, work out the cause and treat it with the help of supplements, dietary modifications and other measures. It is important for pregnant women and those planning to conceive to maintain vitamin B1 levels in a healthy range.