High and Low Lab Results, Explained

A result outside the reference range doesn’t always mean something is wrong. Each page explains what a high or low level means, the most common causes, related symptoms, and which follow-up tests you can order online without a doctor visit. Always share your results with a healthcare provider.

TSH

High TSHA high TSH level (above 4.50 mIU/L, Quest's upper limit) means your thyroid is underactive — hypothyroidism. Your pituitary produces extra TSH trying to stimulate more thyroid hormone. This is the most common thyroid abnormality and is treatable with levothyroxine medication.
Low TSHA low TSH level (below 0.4 mIU/L) means your thyroid is overactive — hyperthyroidism. Your pituitary reduces TSH because it detects excess thyroid hormone. Low TSH requires medical evaluation, as causes range from Graves' disease to thyroid medication overreplacement.

Ferritin

Vitamin D

Low Vitamin DLow vitamin D (below 30 ng/mL) affects an estimated 42% of US adults and causes fatigue, bone pain, muscle weakness, depression, and impaired immunity. It is one of the easiest deficiencies to correct — most people respond well to vitamin D supplementation within 8-12 weeks.

Cortisol

HbA1c

High A1cA high A1c means your average blood sugar has been elevated over the past 2-3 months. An A1c of 5.7-6.4% indicates prediabetes; 6.5% or higher indicates diabetes. Prediabetes is often reversible with lifestyle changes, and diabetes is manageable with treatment.

Vitamin B12

Low B12Low B12 (below 200 pg/mL, or suboptimal below 500 pg/mL) causes fatigue, brain fog, numbness, tingling, and depression. B12 deficiency is common in vegans, older adults, and people taking metformin or acid reflux medications. It is easily correctable with supplementation or injections.

Cholesterol

High CholesterolHigh cholesterol (total above 200 mg/dL or LDL above 130 mg/dL) increases your risk of heart disease and stroke. It causes no symptoms, making lab testing the only way to detect it. Lifestyle changes and statin medications effectively lower high cholesterol.

Testosterone

Low TestosteroneLow testosterone (total T below 264 ng/dL in men) causes fatigue, reduced libido, muscle loss, and mood changes. It affects 4-5 million American men and becomes more common with age. Diagnosis of low testosterone requires two separate morning blood draws confirming low levels.

Estradiol

High EstradiolHigh estradiol outside expected cycle peaks may indicate PCOS, obesity, or liver disease. In men, elevated estradiol causes gynecomastia and reduced libido. Test timing matters, as estradiol naturally peaks at ovulation — test on cycle days 2-5 for an accurate baseline.

Progesterone

Low ProgesteroneLow progesterone (below 1.8 ng/mL on cycle day 21) suggests ovulation may not have occurred or was inadequate. Low progesterone contributes to irregular periods, difficulty conceiving, and early miscarriage. In perimenopause, declining progesterone is expected and drives hot flashes and sleep disruption.

Creatinine

High CreatinineHigh creatinine (above 1.2 mg/dL) may indicate reduced kidney function. Creatinine is a waste product your kidneys filter out, and it accumulates when filtration declines. However, high creatinine also results from high muscle mass, intense exercise, and dehydration, so context matters.

Iron

Low IronLow iron (below 60 mcg/dL) with high TIBC confirms iron deficiency — the most common nutritional deficiency worldwide. Low iron affects energy, cognition, hair growth, and immunity. It can exist months before causing anemia, which is why testing both iron and ferritin is important.

TPO Antibodies

High TPO AntibodiesHigh TPO antibodies (above 35 IU/mL) mean your immune system is attacking your thyroid — a hallmark of Hashimoto's thyroiditis, the leading cause of hypothyroidism. Elevated TPO antibodies signal autoimmune thyroid disease even if your TSH is currently normal.

This page is for informational purposes only and does not constitute medical advice. Consult a healthcare provider for interpretation of your results.