Why these six categories matter
A more precise framework than “everything must be normal”.
In an HRT-FET cycle, the endometrium is central, but it does not function in isolation. Endometrial receptivity, the implantation window, and the tissue response to an embryo are influenced by endocrine status, metabolic health, coagulation risk, immune background, infection status, and uterine anatomy.
Based on ESHRE recommendations on endometrial preparation, ASRM committee opinions, NICE guidance, and Chinese reproductive-medicine consensus statements, pre-cycle screening can be organized into six groups: thyroid function, prolactin, coagulation and thrombosis risk, glucose-lipid metabolism, uterine and endometrial conditions, and infection/immune screening.
Thyroid function
TSH, FT4 and antibodies: the most frequently checked and misread group.
The laboratory reference range for adult TSH is often 0.4–4.0 mIU/L, but fertility care uses stricter thresholds. For women entering an HRT-FET cycle, many guidelines recommend confirming TSH below 2.5 mIU/L and FT4 within the reference range before the cycle starts.
A TSH of 3.5 may be “normal” in general internal medicine and still be treated before assisted reproduction. Both interpretations can be correct because the clinical context is different.
1.1 TSH: a stricter target for ART
Studies associate TSH above 2.5 with lower clinical pregnancy rates and higher early miscarriage risk in IVF/FET populations, although the 2.5–4.0 grey zone remains debated. The practical approach is risk-aware rather than alarmist.
1.2 FT4 and antibodies
- Normal FT4 with elevated TSH suggests subclinical hypothyroidism and usually supports intervention before cycle start.
- Low FT4 despite normal TSH requires endocrine evaluation; it should not be ignored.
- Positive TPO-Ab or TG-Ab with normal TSH and FT4 indicates autoimmune thyroiditis, but not necessarily a reason to postpone the cycle.
1.3 Thyroid antibodies
The TABLET trial showed that levothyroxine did not improve live birth in euthyroid women with positive TPO antibodies. The mainstream interpretation is that TPO-Ab positivity alone, when TSH is below 2.5 and FT4 is normal, can usually proceed with monitoring.
| Dimension | Red | Amber | Clear |
|---|---|---|---|
| TSH | >10 mIU/L: treat first | 2.5–10 mIU/L: usually treat or individualize | <2.5 mIU/L |
| FT4 | Below range: endocrine review | Near lower limit: interpret with TSH | Normal |
| TPO/TG-Ab | — | Positive with high TSH | Positive but TSH normal: monitor |
Prolactin
A high number does not automatically mean a pituitary tumor.
Mild hyperprolactinemia is commonly caused by stress, recent sexual activity, nipple stimulation, or medications such as antidepressants, antiemetics, or antihypertensives. Macroprolactinemia is another common cause of an apparently high result with limited biological activity.
2.1 Stratified handling
25–50 ng/mL: repeat in the early morning, fasting, and in a calm state. Review medications and physiological causes.
50–100 ng/mL: assess macroprolactin using PEG precipitation when available; unnecessary treatment can often be avoided.
>100 ng/mL: pituitary adenoma becomes more likely. Pituitary MRI and endocrinology care are recommended before proceeding.
2.2 HRT itself can raise PRL
Estrogen used during HRT stimulation can increase prolactin. Therefore PRL measured after HRT begins is often physiologically higher. Entry assessment should rely on baseline early-follicular results before hormone use.
Coagulation and thrombosis risk
An HRT cycle is itself a thrombosis-risk event.
Exogenous estrogen is a known thrombosis risk factor. Before an HRT cycle, coagulation screening is not optional in higher-risk patients; it is part of safety assessment.
- PT, APTT, TT and fibrinogen identify obvious coagulation abnormalities.
- D-dimer reflects fibrinolytic activity. Mild elevation is common; persistent marked elevation needs explanation.
- Antiphospholipid antibodies are essential when there is recurrent miscarriage, prior IVF failure, thrombosis history, or autoimmune background.
- Homocysteine elevation is relevant to thrombotic and early pregnancy risk and can often be corrected with folate, B6 and B12.
3.1 APS is a hard red flag
Confirmed antiphospholipid syndrome should be treated and stabilized before an HRT cycle. Low-titer single positivity is not the same as APS; diagnosis requires repeat positivity at least 12 weeks apart plus clinical criteria.
3.2 D-dimer is often overinterpreted
A value just above the reference range, such as 0.6 mg/L when the upper limit is 0.5, is usually not a reason to cancel a cycle. Persistent values above roughly 1.5 mg/L, especially with symptoms or risk factors, require work-up.
Glucose and lipid metabolism
BMI, glucose and insulin resistance are the soil of the endometrial environment.
Metabolic abnormalities may not cancel a cycle immediately, but they can affect endometrial inflammation, estrogen pharmacokinetics, thrombosis risk, and pregnancy safety.
- BMI ≥35 kg/m² is usually a relative contraindication and supports weight intervention before ART.
- HbA1c ≥6.5% or fasting glucose ≥7.0 mmol/L indicates diabetes that should be controlled before treatment.
- HOMA-IR above 2.5 suggests insulin resistance; metformin and lifestyle intervention may be considered before HRT.
- Marked LDL-C elevation or familial hypercholesterolemia needs cardiovascular evaluation.
Uterus and endometrial conditions
The main stage of the HRT cycle must be prepared before the embryo arrives.
Uterine assessment should include transvaginal ultrasound and, when indicated, hysteroscopy or MRI. Polyps, submucosal fibroids, adhesions, uterine malformation, cesarean scar niche, and hydrosalpinx-related fluid can all reduce implantation probability.
Most centers use endometrial thickness of 7 mm as a minimal line, 8 mm as a more reassuring threshold, and 9 mm or more as an ideal state before progesterone conversion. Thin endometrium is not always predictable before stimulation, but a history of thin lining should trigger planning.
Infection and immune screening
The key is to distinguish hard exclusions from over-treatment.
Baseline infection screening should include syphilis, HIV, HBV, HCV, HPV and Chlamydia trachomatis. TORCH baseline status is also useful; rubella IgG-negative patients should complete vaccination and wait at least one month before proceeding.
6.1 Chlamydia is a red light
Active chlamydial infection can cause subclinical endometritis, tubal injury, pelvic inflammatory disease and persistent inflammatory activation. It should be treated and confirmed negative before HRT-FET.
6.2 Mycoplasma needs three-way interpretation
- Mycoplasma genitalium: clearly pathogenic; treat and confirm clearance.
- Ureaplasma: often colonization; asymptomatic positivity is not a routine reason to delay FET.
- Mycoplasma hominis: conditionally pathogenic; symptoms and inflammation guide treatment.
Repeated antibiotics aimed only at making asymptomatic Ureaplasma PCR negative can damage the vaginal microbiome and may create more problems than benefits.
6.3 Immune tests
Reasonable baseline immune screening is usually limited to ANA, thyroid antibodies, and antiphospholipid antibodies. NK-cell activity, Th1/Th2 ratios and broad cytokine panels are not supported as routine pre-HRT tests.
Integrated decision-making
Put the six categories on one table.
No single index should decide cycle entry in isolation. A more useful approach is to classify all findings as red, amber, or clear. A single amber result among otherwise reassuring findings may not change timing; a red result must be treated first.
Another crucial distinction is baseline versus in-cycle testing. PRL, TSH, coagulation and lipids can shift once estrogen and progesterone are used. Entry decisions should be made from baseline reports, while in-cycle results are used for monitoring.
References
- ATA Guidelines, Thyroid, 2017.
- ESHRE good practice recommendations on recurrent implantation failure, 2023.
- ESHRE recommendations on endometrial preparation for FET, 2024.
- Endocrine Society Clinical Practice Guideline on hyperprolactinemia, 2011.
- TABLET Trial, New England Journal of Medicine, 2019.
- ASRM committee opinions on subclinical hypothyroidism, obesity and reproduction.
- NICE CG156 Fertility problems: assessment and treatment.
- WHO and CDC STI guidelines; Chinese reproductive medicine consensus statements.
Need help interpreting HRT-cycle screening results?
This article is for education and clinical communication only. Cycle entry and medication decisions should be made by a reproductive physician with full records.