Menstrual cycle and lab test results: what changes depending on the phase

«Menstrual cycle and lab results» is a search that usually starts with the report in hand: a progesterone flagged as low, an FSH in bold or a cholesterol different from last year. Many of those numbers depend on the cycle day the sample was taken, and a blood test done on the wrong day can look abnormal without being so.^[3][4] Here you will see what the hormones do in each phase, which tests have a preferred day, what changes only a little and what to do if your cycles are irregular or you use hormonal contraception.

What happens in a cycle and why a result changes

The menstrual cycle is the monthly process in which hormones stimulate an ovary to release an egg, the lining of the uterus thickens for a possible pregnancy and, if there is none, it is shed with the period.^[1] The average is 28 days, but it varies between women and from month to month: for most women it lasts 21 to 35 days.^[1] Days are counted from the first day of bleeding, which is day 1, and the period lasts about five days on average.^[1]

Within that month the hormones follow a predictable path. When bleeding ends, around day 5, estrogen starts to rise and thickens the uterine lining.^[1] Ovulation can happen anywhere between days 10 and 21 counted from the start of the period, and body temperature rises slightly with it.^[1] Around ovulation LH surges: in the reference ranges of a UK public health system laboratory, its range goes from 1 to 12 IU/L in the follicular phase to 16 to 104 IU/L in the ovulatory phase.^[3] Progesterone is below 1 ng/mL before ovulation and reaches 5 to 20 ng/mL mid-cycle.^[6] If there is no pregnancy, hormone levels fall and that drop signals the uterus to shed its lining.^[1]

The practical consequence: the same woman, in the same month, can have an LH several times higher one day than another without anything being wrong.^[3] That is why laboratories give ranges by phase, and a hormone result without the date of the period is only half a data point. It is a clear example of why the lab range is not your personal norm.

Marker by marker: when to test in the cycle and what distorts it

Not every test has a preferred day. Some are only read well in a specific phase; others depend more on bleeding, stress or contraception. This table sorts what the sources of this text say:

«Day 21» only works in 28-day cycles

Progesterone is the clearest example of a badly timed test. What is sought is the midpoint of the luteal phase, and a UK public health system laboratory defines it like this: the sample is taken 7 days before the start of the period, for example on day 21 of a 28-day cycle or on day 28 of a 35-day one.^[4] If your cycles last 35 days and it was drawn on day 21, the sample was taken a week before the measuring point, and a low value may be due to that alone. The same laboratory asks for the cycle day to be stated on the request, when known.^[4]

Since the next period is not known until it arrives, you count backwards: note the day of the draw and, when your period comes, count the days. If it was about seven, the sample was on point. With irregular or long cycles, the reference suggests measuring progesterone weekly until the period starts.^[4]

FSH, LH and estradiol: the snapshot of the first days

FSH and LH are interpreted mainly with samples from the first days of the cycle, for example when the ratio between the two is looked at.^[3] When the question is whether ovarian function ended early in a woman under 45, the reference asks for two samples 4 to 6 weeks apart, without hormonal contraception, before giving weight to a high FSH.^[3] By contrast, in women over 45 with typical symptoms, menopause is recognized clinically and FSH and LH are not routinely recommended for that.^[3] Estradiol changes with the time of month, so its result is only read with the cycle day noted.^[5]

What does a progesterone of 3, an FSH of 8 or an LH of 40 mean?

The number alone does not decide. Each laboratory has its own range, some use other measuring methods, and the source itself asks you to talk with your doctor about what your specific result means.^[5][6] The table uses the ranges from the sources of this text so you can see how the reading changes with the day; if your report shows another range, your lab's range rules.

What changes only a little: iron, ferritin and cholesterol

With iron, the useful question is not which day the sample was taken but how much you bleed each month. Heavy periods are one of the causes of iron-deficiency anemia, and to detect it doctors check the complete blood count, hemoglobin, blood iron and ferritin; low iron and low ferritin point in that direction.^[9] Many people with mild or moderate anemia have no symptoms; in more marked cases there is tiredness, shortness of breath, lightheadedness or pale skin.^[9] If your ferritin came back low, in this text on low ferritin we explain which tests go with it, and in how to read the complete blood count, which line to look at.

Bleeding is considered heavy when it lasts more than eight days or soaks through one or more pads or tampons every one to two hours, and it affects one in five women in the United States each year.^[2] That is why persistent tiredness and hair loss with heavy periods deserve a look at iron, as we cover in tired all the time and in hair loss.^[9]

Cholesterol does move with the cycle, though only a little. In the BioCycle study, lipids were measured in 259 healthy women aged 18 to 44 with regular cycles up to eight times per cycle: total and LDL cholesterol came out lower in the luteal phase than in the follicular phase, and HDL was highest around ovulation.^[8] More women exceeded desirable levels, LDL of 130 mg/dL or more or total cholesterol of 200 mg/dL or more, when measured in the follicular phase.^[8] For triglycerides, the study abstract reports no difference by phase.^[8] This is not about choosing the day that looks best: if your cholesterol is close to a cut-off, compare results from the same phase and note which it was. In high cholesterol we explain what matters more than the cycle day.

What has no cycle day

TSH does not appear in these sources with a preferred day; it enters this conversation because thyroid problems are among the causes of irregular cycles.^[2] How to read it is in thyroid panel: TSH, T4 and T3. For glucose and the chemistry panel, preparation matters more than the phase: we explain it in fasting before a blood test.

Hormonal contraception and other things that move the result

Hormonal contraception replaces the signals of the natural cycle, and that changes what a result means. According to the UK reference, FSH is of no value for assessment in women using combined estrogen and progestogen contraception or a high-dose progestogen.^[3] Birth control pills are also among the medicines that can alter estradiol, together with estrogen therapy, corticosteroids and testosterone; your doctor may ask you to pause one temporarily before the test, but do not stop any without talking it over first.^[5] Estrogens are also among the medicines that raise prolactin.^[7]

Other things that change the reading:

• Pregnancy: progesterone moves to much higher ranges, 11.2 to 90 ng/mL in the first trimester,^[6] and the prolactin reference limit rises from 25 to 500 ng/mL.^[7]
• Stress: physical or emotional stress occasionally raises prolactin,^[7] and high chronic stress can make cycles irregular.^[2]
• Recent exercise before the draw: it raises prolactin.^[7]
• Large weight changes: gaining or losing a lot of weight can cause missed periods.^[2]
• Age: in adolescence and perimenopause irregular cycles are normal, and hormone ranges change after menopause.^[2][3]
• The laboratory: ranges and methods vary between laboratories; to compare, use the same one.^[6]

If your cycles are irregular or have stopped

US women's health sources advise seeing a doctor when your period comes more often than every 24 days or less often than every 38, when three months pass without a period while not pregnant or breastfeeding, when bleeding lasts more than eight days or when it soaks one or more pads every one to two hours.^[2] There are stages when irregularity is normal, such as adolescence and perimenopause.^[2] Common causes include chronic stress, large weight changes, thyroid problems and polycystic ovary syndrome, which is behind about one in ten women with irregular cycles.^[2]

With irregular cycles, calendar rules stop working. There is no «day 21» to use: the UK reference suggests measuring progesterone weekly until the period arrives.^[4] The LH to FSH ratio in samples from days 1 to 5 is often raised in women with polycystic ovaries, but it can also be normal, and for that syndrome the reference requires two of three criteria and ruling out other causes, an assessment that belongs to your doctor.^[3] No single result on this page is enough to put a name to what is happening.

What you can do is arrive with the calendar: every first day of bleeding, how many days it lasts and how heavy it is. Since body temperature rises slightly with ovulation, logging it on waking helps show whether there was a mid-cycle shift.^[1]

What to do: note the phase and when to repeat

Almost every «odd» result caused by the cycle is solved with two dates and a well-timed repeat:

1. Before each draw, note the first day of your last period. That tells you which cycle day you are on.^[1]
2. Ask for the cycle day to appear on the request. Laboratories ask for it in order to interpret.^[4]
3. FSH, LH and estradiol: in the first days, if your doctor wants the snapshot of the early follicular phase.^[3]
4. Progesterone: count backwards. About 7 days before your next expected period, based on the real length of your cycles.^[4]
5. After the draw, note when the next period came. That date confirms whether progesterone was on point.^[4]
6. Cholesterol: repeat in the same phase as last time to compare without the cycle effect.^[8]
7. Iron and ferritin: note how the bleeding was that month and whether the draw was during or after the period.^[9]

When to repeat: a high FSH before age 45, with a second sample 4 to 6 weeks later;^[3] a high prolactin, early in the morning while fasting;^[7] a progesterone off its point, in the next cycle and on the right day.^[4] Everything else goes to the next check-up: the list by age is in what to include in the annual check-up and the general method in how to read your lab results without panicking.

Warning signs

Almost everything in this text is solved with a scheduled appointment. This does not wait:

• Sudden, intense pain in the abdomen together with strong dizziness or fainting, nausea or marked paleness: call emergency services or go to the emergency room immediately, especially if you could be pregnant.^[10]
• Those same symptoms, even if milder, when you could be pregnant, even without a positive test: contact your doctor the same day.^[10] A pregnancy outside the uterus is a medical emergency.^[2]
• Chest pain or trouble breathing during or after your period: seek medical care; if it is an emergency, call emergency services.^[2]
• Bleeding that soaks one or more pads or tampons every one or two hours or lasts more than eight days: see a doctor soon.^[2]
• Three months without a period while not pregnant or breastfeeding, or cycles shorter than 24 days or longer than 38: book an appointment.^[2]

What to bring to the doctor

An appointment about your cycle or a hormone result is decided by what you bring:

1. A calendar of your cycles for the last three to six months: first day of each period, how many days it lasted and how heavy it was.^[2]
2. Each result with the cycle day it was taken on and the date of the period that came next.^[4]
3. A list of hormonal contraceptives and medicines, including corticosteroids, estrogens and antidepressants, and any hormonal supplement.^[5][7]
4. Previous results for the blood count, ferritin and lipid panel, with the phase noted if you know it.^[8][9]
5. Temperature on waking, if you log it, to see whether there is a mid-cycle shift.^[1]
6. Your questions: on which day of my cycle should this test be repeated, which other test would help, which hypothesis is worth checking. In health hypotheses we explain how to build one.