Written by &you Labs Team
Updated September 17, 2026
Depression is mostly a mental health condition. Doctors diagnose it by your symptoms. They look at how long these last and how much they affect your life. But low mood can also have a real, physical cause hiding in your blood. Fixing that cause often helps, alongside your mental health care, not instead of it.
Think of this guide as a partner to mental health treatment. It is not a replacement for it.
Low mood can have a measurable cause. The most important one to rule out is low thyroid function. Low vitamin D and low B12 can play a role too. Low iron and low omega-3 can too. So can hormone shifts, blood sugar problems, and ongoing inflammation. Blood tests do not diagnose depression. They find fixable causes that often improve your mood when treated.
Depression is a clinical diagnosis. Blood tests play three roles here:
See a mental health professional right away if you feel deep sadness or hopelessness. This includes any thoughts of suicide. Do not wait for blood test results first.
This is the most important one to rule out. Look for a high TSH, especially above 4.5. Look for a low or borderline Free T4 and Free T3 too. Low thyroid function causes fatigue, low mood, brain fog, weight gain, and feeling cold. It gets mistaken for depression for months sometimes. Treating it often lifts your mood in a real way.
Look for a level under 30 ng/mL. Studies link low vitamin D to depression. Fixing it helps mood in people who were actually low. A dose of 2,000 to 4,000 IU a day for 12 weeks is a common approach.
Look for a level under 500 pg/mL. This matters especially with a high homocysteine. Low B12 can affect mood, on top of thinking and physical symptoms. A B12 supplement usually helps.
Look for a ferritin under 50 ng/mL in women. Low iron often brings fatigue and low mood together. Iron supplements help.
Look for an omega-3 index under 8%. The evidence for mood benefit is mixed. But some people respond well, especially with a higher EPA dose. Try 1,000 to 2,000 mg of combined EPA and DHA daily. Eat fish on top of this.
Look for a level under 9 ng/mL, especially with a high homocysteine. A gene variant called MTHFR can cause a subtle version of this problem. A methylated folate supplement, called 5-MTHF, has some evidence. It works as an add-on for depression treatment.
Perimenopause brings swinging estradiol and falling progesterone, which often brings mood symptoms. Low testosterone in men links to low mood and low motivation. The months after giving birth bring hormone shifts on top of poor sleep. PMS and PMDD bring mood symptoms tied to your cycle.
Look for an HbA1c between 5.5 and 6.4%, or a high fasting insulin. Blood sugar swings link to mood swings in some people. Diet and lifestyle changes help.
Look for an hs-CRP that stays above 2 mg/L. Inflammation affects mood through chemical signals sent to your brain. This is a growing area of mood research. Fixing what drives the inflammation tends to help.
An unusual cortisol pattern often links to chronic stress. This can bring low mood too. Managing stress and sleep helps. A detailed cortisol curve test can help map the pattern too.
Blood tests cannot diagnose major depressive disorder. They cannot diagnose dysthymia, a longer-lasting form of low mood, either. They cannot diagnose bipolar disorder, seasonal mood changes, postpartum depression, or an adjustment disorder. A psychiatrist, psychologist, or trained mental health professional makes these diagnoses.
Low thyroid function is the single most important condition to rule out. Here is what research shows:
You can read more in our full guide to thyroid testing.
This deserves special attention. It affects roughly 1 in 8 to 1 in 10 women after birth.
Mood symptoms during perimenopause are common, and often brushed aside. Estradiol swings while progesterone falls.
Want a full biomarker check alongside your mental health care? Book your &you Labs panel and see what your results show.
A &you Labs panel covers the main biomarker causes of low mood. This includes a full thyroid panel, B12, folate, vitamin D, and ferritin. It also covers HbA1c, hs-CRP, homocysteine, hormones, and cortisol, where needed. Your report suggests fixing what it finds. This works alongside your mental health care, not instead of it.
For most people with primary depression, no, but it often helps meaningfully. It supports how well therapy and medicine work. For some people, fixing the cause can make a huge difference. This is especially true with undiagnosed low thyroid function.
A basic check makes sense before starting treatment. This covers thyroid, vitamin D, B12, and ferritin. This should never delay care if your symptoms are severe.
A full thyroid panel is often the most useful single addition. This covers TSH, Free T4, and Free T3. It catches low thyroid function, a treatable mimic of depression.
There is modest evidence for benefit, mainly in people who were actually low. The effect supports mood, it does not cure depression on its own. It is worth fixing regardless.
No single blood test diagnoses depression. Some branded panels get marketed this way. But they lack strong proof for routine use. A standard biomarker workup has more evidence behind it.
The &you Labs Team combines medical science with personal care. We help Filipinos understand their health markers with blood work analysis that is easy to follow, whole person support, and health expertise built for Filipino life.
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