Pill and Blood Values: 9 Biomarkers That Change

4. Testosterone: Reduced by up to 50 percent

Testosterone plays an important role in women's libido, muscle strength, bone density, and energy metabolism. Combined oral contraceptives lower testosterone levels through three simultaneously acting mechanisms: Suppressed ovarian function reduces ovarian androgen production. Increased SHBG levels bind more circulating testosterone, making less free testosterone available. And the adrenal cortex, which produces about 25 percent of testosterone in women, also has its activity throttled.

A systematic review and meta-analysis by Zimmerman et al. (Human Reproduction Update, 2014), which evaluated 42 studies, showed a significant decrease in total testosterone in combined contraceptive users (mean difference: -0.49 nmol/L). Since free testosterone decreases even more due to the simultaneous increase in SHBG, the effect on the biologically active hormone can be even greater.

For the interpretation of your lab values, this means: Low-normal or low testosterone in a pill user is often a direct pill effect. Nevertheless, it is worthwhile to address symptoms such as loss of libido, persistent fatigue, or reduced resilience, as these effects are clinically relevant for some women.

5. Cortisol: Altered Stress Response

Testosterone acts as a natural antagonist to the stress hormone cortisol. When testosterone decreases due to the pill, this balancing effect weakens. In addition, synthetic estrogens stimulate the liver to increase the production of corticosteroid-binding globulin (CBG), which influences cortisol regulation in the body.

A meta-analysis in Psychoneuroendocrinology (2022) found altered cortisol reactivity in women on oral contraception in standardized stress tests. Another study in Brain, Behavior and Immunity (2024) showed higher inflammation and stress levels in OC users under acute social stress. The magnitude of these effects varies widely between individuals.

In blood tests, this can mean: Slightly elevated basal cortisol levels can be an indirect pill effect and do not necessarily indicate an independent adrenal gland disorder. However, symptoms such as increased irritability, sleep problems, or reduced stress tolerance are a reason to discuss this issue with a doctor.

6. Progesterone: No increase in the second half of the cycle

In the natural cycle, progesterone rises significantly after ovulation. The corpus luteum hormone is produced by the corpus luteum, prepares the uterine lining for possible implantation, and has a direct effect on the nervous system, including a calming, sleep-promoting, and mood-regulating effect via the GABA receptor.

Hormonal contraception suppresses ovulation. No corpus luteum without ovulation, no natural progesterone release without a corpus luteum. Studies show that women on combined hormonal contraception consistently have lower progesterone levels than women with a natural cycle in the luteal phase.

For blood value interpretation, this means: Progesterone measured in a pill user at any time is not comparable to the value of a naturally menstruating woman. Statements about corpus luteum function or the luteal phase are generally not possible under hormonal contraception, as this cycle phase does not occur.

7. hsCRP: Inflammation marker in the elevated range

High-sensitivity C-reactive protein (hsCRP) is a sensitive marker for subclinical inflammatory processes in the body. Values below 1 mg/L are considered favorable, while values above 3 mg/L are clinically relevant for cardiovascular risk.

Synthetic estrogens in combined contraceptives stimulate the liver to increase CRP production. A study by Cauci et al. (Sports Medicine, 2017) examined 205 female athletes and found hsCRP values above 3 mg/L in 26.4 percent of pill users, compared to only 2.6 percent of women not using contraception (Odds Ratio 13.3; p < 0.001). This effect has not been documented with comparable strength for progestogen-only methods.

For evaluation in a blood count: Slightly elevated hsCRP values in women using combined hormonal contraception can be a direct estrogen effect on the liver and do not necessarily indicate an inflammatory disease process. Persistently high values should still be medically investigated.

8. Vitamin B12: Subclinical effects possible

Vitamin B12 is essential for nerve health, blood formation, and DNA synthesis. Several studies have investigated a link between oral contraception and lower serum vitamin B12 levels. A review in Clinical Chemistry and Laboratory Medicine (2023) documented measurable changes in micronutrients, including vitamin B12, with hormonal contraception.

The effect size should be evaluated differentially: In most healthy women using modern low-dose pills, B12 changes are small and remain within the reference range. A relevant deficiency is primarily likely when additional risk factors are present: plant-based diet, stomach acid deficiency, already low baseline levels, or long duration of use.

Recommendation: If you are taking the pill, checking your B12 level may be advisable. Holotranscobalamin (active B12) is more informative than total serum B12. Supplementation with methylcobalamin or hydroxocobalamin may be beneficial in cases of confirmed deficiency.

9. Cholesterol and triglycerides: Varying depending on the method

Hormonal contraception influences the lipid profile, and this varies depending on the method.

Combined hormonal methods with synthetic estrogen (combined pill, vaginal ring, contraceptive patch) typically increase HDL cholesterol and simultaneously raise triglyceride levels. LDL cholesterol often remains neutral or decreases slightly. The increase in triglycerides is particularly relevant for women with a genetic predisposition to lipid metabolism disorders.

Progestogen-only methods (hormonal IUD, implant, mini-pill, three-month injection) can, however, lower HDL and, with certain preparations, slightly increase LDL. This is due to the partially androgenic effect of some progestogens. Women with familial hyperlipidemia or elevated baseline triglyceride levels should choose their contraceptive method in consultation with their doctor and have their lipid profile checked regularly.

What you should know before your next blood test

If you are using hormonal contraception and having lab values determined, several points significantly improve interpretation:

• Always state which contraceptive method you are using: the type of method (combined or progestogen-only), the specific preparation, and the duration of use influence the interpretation of almost all hormone and inflammatory parameters.
• For an unbiased hormone status, a break is ideally necessary: A baseline measurement of DHEAS, free testosterone, estradiol, or progesterone without the pill's effect should be taken at least two to three months after discontinuation.
• Elevated SHBG is expected with the combined pill and is not a sign of illness. It becomes clinically relevant only in combination with symptoms.
• hsCRP, lipids, and B12 deserve special attention in long-term users and with co-existing risk factors.
• Progesterone and estradiol levels during hormonal contraception are not comparable to those in naturally menstruating women.

Conclusion

Hormonal contraception affects much more than just the menstrual cycle. At least 9 laboratory values can measurably and sometimes significantly change: DHEAS and testosterone decrease, SHBG increases by up to 200 percent, estradiol and progesterone are suppressed, hsCRP and triglycerides can increase, and vitamin B12 status deserves special attention.

The practical consequence of this is that blood tests must always be interpreted in the context of the contraceptive method. What appears as low testosterone or elevated SHBG in a pill user is often not a pathological finding, but a direct measurable effect of contraception. Conversely, symptoms such as persistent fatigue, loss of libido, or mood changes should not be dismissed solely based on laboratory results.

Anyone who wants a complete and undistorted hormone status should ideally plan the measurement after a break of several months from the combined pill and discuss this consideration openly with their doctor.