13 Labs I Order for Every Patient Under 40. Print This for Your Next Appointment.

Sara Szal MD

She was 33, training for a half marathon, arriving early to a job she was good at, and tired in a way none of her friends seemed to be. Her doctor ordered basic labs, told her the results fell within range, and sent her home.

Ostensibly, nothing was wrong. No disease had declared itself, and no organs were in trouble. But no one commented on whether those numbers were normal for her because no one had measured her baseline.

That patient was me.

I was 33 the first time a doctor called a blood draw my baseline, and it wasn’t one. A baseline is useful because it gives you something to compare later. Mine was a single point with no history behind it.

Insulin resistance, when your cells become numb to insulin and you accumulate fat, does not always announce itself. I had to dig for it. I was a physician yet none of my doctors were checking for it. Low iron can feel like overwork and not be detected in basic labs. Again, my low iron wasn’t detected until I ordered my own ferritin level. Thyroid changes can be mistaken for stress, like mine were. Sometimes the first clue is a symptom pattern, sometimes it is a lab result, and often it is both.

What I was taught in medical training: most non pregnant young women don’t need lab testing, according to the US Preventive Services Task Force. Done.

What I understand now, more than a decade and thousands of patients later: there are actionable results that are important to consider in women under 40, particularly with low iron in one in three women in the United States, autoimmune thyroiditis (antibodies can be detected in 13% of adults), and loss of ovarian reserve. One result without context is not the whole story. Trends, symptoms, family history, genetics, medications, cycle timing, life stage, and specifics about women’s health (like ferritin levels) all change what a number means.

I believe every woman under 40 deserves a thoughtful baseline conversation before symptoms become impossible to ignore and harder to treat. That does not mean every woman needs every test below. It means she should know what question each test answers and why it may, or may not, belong in her care.

I have run versions of these markers on myself more than once since that first appointment. Some moved. Some did not. The value was not the stack of results; it was knowing which changes mattered in context.

Your routine check-up, with a standard metabolic panel and lipid panel when appropriate, is not wrong. It is foundational. This list adds possible questions though it does not replace medical judgment.

This is not about collecting data for the sake of it. It is about owning enough of your health story to recognize a meaningful change. Your chart belongs to you before it belongs to any system that stores it.

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A single normal number is a snapshot, not a story. A useful baseline begins with a question and becomes more valuable when it can be compared over time.

Here is the sentence to bring with you to your next appointment: “I’d like to understand what a useful baseline would look like for me, based on my symptoms, family history, medications, menstrual pattern, and future plans. Which tests would meaningfully change what we do next?”

You are allowed to ask that question. You are also allowed to ask what a test can tell you, what it cannot tell you, and what would happen differently if the result came back high, low, or in range.

Save this list, bring it to your next appointment, and ask which items fit your history. The goal is not necessarily to leave with 13 orders. The goal is to leave knowing which questions matter for your body now and which labs may provide an important baseline.

1. Fasting glucose. A widely used snapshot of blood sugar after fasting. It can help screen for diabetes and prediabetes, but it is only one moment in time. Optimal levels are 70 to 85 mg/dL.

2. Hemoglobin A1c. An estimate of average blood sugar over roughly two to three months. Anemia, recent blood loss, pregnancy, kidney disease, and some hemoglobin variants can affect how it should be interpreted. I aim to keep it < 5.5%.

3. Fasting insulin. While it’s not a universal screening test and not standardized as neatly as glucose or A1c, it may adds context in many cases. It can change 10 or more years before your fasting glucose.

Note: I use a continuous glucose monitor in nearly every patient so that we can personalize the food plan and improve metabolic health. Since more than 90 percent of the US population has at least one criteria for metabolic syndrome, these are very important baseline tests.

For readers of Asian ancestry: Diabetes risk can appear at a lower body mass index. U.S. guidance considers a lower screening threshold for Asian American adults, while local guidance varies across countries. Family history, prior gestational diabetes, and PCOS also belong in the conversation.

4. Advanced lipid panel. Total cholesterol, LDL-C, HDL-C, and triglycerides provide too basic of a starting point. From this, you can calculate your cholesterol/HDL ratio. Under 40, I recommend adding a few advanced tests such as ApoB (a direct particle count, more precise than LDL alone) and lipoprotein(a), a largely genetic marker that only needs to be checked once before perimenopause and menopause, when it can rise. Cardiovascular risk is not determined by one cholesterol value alone, so blood pressure, smoking, diabetes, family history, and age still matter.

5. High-sensitivity C-reactive protein (hs-CRP). A nonspecific inflammation marker that can help refine cardiovascular risk in selected people. It can rise with infection, injury, and other inflammatory conditions, so context matters.

6. Ovarian reserve. This is controversial, but I test anti-mullerian hormone (AMH) and day 2 to 4 estradiol and FSH in every patient. Together these three markers give a fast, low-cost read on ovarian reserve — the size of the remaining follicle pool and how the ovaries will respond to stimulation — which directly changes fertility counseling and treatment timing in women under 40.

   • AMH is the most direct serum marker of the primordial follicle pool, can be drawn on any cycle day, and is the best single predictor of ovarian response to gonadotropins.

   • Day 2–4 (basal) FSH is highly specific for diminished ovarian reserve when clearly elevated (>10–15 mIU/mL) — a rising FSH signals the ovary needs more drive to mature a follicle.

   • Early-follicular estradiol must be drawn alongside FSH: an early estradiol rise can suppress FSH into the normal range, producing a falsely reassuring value, so estradiol is what lets you interpret the FSH correctly.I was taught to only do this in women seeking fertility who are unable to conceive for a year (<35) or in 6 months (35-40).

   • Reserve markers below age-specific thresholds (e.g., AMH <1 ng/mL, FSH >10 IU/L, estradiol <80 pg/mL) flag a woman whose reproductive window may be closing faster than expected. Let me know in the comments if you want more information about these blood tests.

7. TSH, with free T3 and T4, and thyroid antibodies. TSH is commonly used as the first thyroid test. Free T3 gives you the biological active form of thyroid hormone, and together with free T4, may add additional context when TSH is abnormal or when symptoms, medications, pregnancy, or pituitary concerns change the question. Thyroid antibodies (ask for thyroid peroxidase antibodies and anti-thyroglobulin levels) tell you about autoimmune thyroiditis. Recent testing shows that about 13 percent of people have thyroid antibodies, and it’s better to know early before Hashimoto’s thyroiditis causes damage to the thyroid.

8. Vitamin D (25-OH). Most useful when symptoms or risk factors make deficiency more likely. I recommend routine population-wide screening because vitamin D is a nutrient and a hormone that is involved in 300+ jobs in the body, including supporting the integrity of your gut lining and keeping your bones strong through your 20s and 30s.

9. Vitamin B12. Worth discussing with persistent fatigue, neurologic symptoms, a vegan diet, gastrointestinal disease or surgery, metformin use, or long-term acid-suppressing medication.

10. Ferritin and iron studies. Especially relevant with heavy menstrual bleeding, pregnancy, endurance training, restless legs, hair shedding, shortness of breath, or fatigue. Ferritin can also rise with inflammation, so it should not be read alone. My ferritin was less than 10 µg/L for most of my 30s, and no one was tracking it but me. I like to see it around 50 to 80 µg/L in women during their reproductive years. One in three women in the United States and one in two women in the United Kingdom are too low. Not all of us absorb enough iron orally to make up for menstrual losses, myself included.

Note: If you are in your 20s to 30s and exhausted in a way that sleep does not fix, do not assume the only explanations are work, caregiving, motherhood, stress, or getting older. Fatigue deserves a history first, then targeted testing.

11. Comprehensive metabolic panel. A group of tests that can provide information about electrolytes, glucose, kidney function, liver enzymes, and proteins. I recommend it to screen for fatty liver (ALT>20 in women and >30 in men) and total protein.

12. Complete blood count with differential. Measures red cells, white cells, and platelets. It can help evaluate anemia, infection, and some blood disorders, but an in-range CBC does not rule out every cause of fatigue. It can help to confirm iron-deficiency anemia. You can also calculate the ratio of neutrophils-to-lymphocytes to assess for inflammation (normal is a ratio <1).

13. Urinalysis. Useful for selected urinary, kidney, pregnancy-related, or metabolic questions. It is not automatically necessary for every healthy, asymptomatic adult.

One practical detail for international readers: Units and reference ranges vary by country and laboratory. Keep the full report, including the unit and range, rather than saving only a note that says normal.

You will want to start a time series of your labs over time. Knowing what your baseline labs are when you are in your 20s and 30s will help you navigate perimenopause after 40 with more awareness and precision.

May you always be the first to know your own body. Find a collaborative physician who is willing to measure your baseline and interpret the results with you.

xo, Dr. Sara

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Medical disclaimer: This article is educational and is not a diagnosis, a universal laboratory order, or a substitute for individualized care. Testing should be based on symptoms, history, medications, reproductive plans, local guidance, and a clinician’s judgment.

Notes

Auerbach M et al. Iron Deficiency in Adults. JAMA, 2025.

Santoro N, Polotsky AJ. Infertility Evaluation and Treatment. NEJM, 2025.

Tawfik YMK et al. Absolute and Functional Iron Deficiency in the US, 2017-2020. JAMA Open Network, 2024.

U.S. Preventive Services Task Force: Prediabetes and Type 2 Diabetes Screening — Includes the lower BMI screening consideration for Asian American adults. Local recommendations may differ.