Venous Thromboembolism and Hormone Replacement Therapy
Background
RCOG withdrew this guideline and now directs clinicians to NICE NG23 for current recommendations on menopause and HRT. However, the core pharmacological principle the guideline established — that VTE risk with HRT is route-dependent, not a fixed class effect — remains the standard teaching point and has been carried forward largely unchanged into NICE guidance and subsequent evidence (e.g. the QResearch/CPRD case-control studies).
Route of Administration and VTE Risk
- Oral HRT (estrogen ± progestogen) increases VTE risk roughly 2–3-fold above baseline population risk. Large nested case-control data give an odds ratio of approximately 1.58 (95% CI 1.52–1.64) for oral HRT users versus non-users.
- Transdermal HRT (patches, gels) at standard therapeutic doses carries no significant increase in VTE risk over baseline — OR approximately 0.93 (95% CI 0.87–1.01) in the same data. This is because transdermal delivery avoids first-pass hepatic metabolism, which is what drives the procoagulant effect of oral estrogen (increased hepatic synthesis of clotting factors, reduced antithrombin/protein S).
- Risk is highest in the first 1–2 years of use for oral preparations and attenuates with combined regimens depending on progestogen type.
- Progestogen choice matters: micronised progesterone and dydrogesterone are considered to carry little or no added thrombotic risk compared with other synthetic progestogens when combined with estrogen.
Practical Guidance
- Women with increased baseline VTE risk (BMI >30 kg/m², strong family history of VTE, known hereditary thrombophilia, or a personal history of VTE) should be offered transdermal rather than oral HRT.
- Women with a strong family history or known thrombophilia should be referred for haematology assessment before starting HRT, regardless of route.
- HRT is not an absolute contraindication in women with a past VTE — the route of administration is the key modifiable factor, not whether HRT is used at all.
High-Yield Exam Points
- Transdermal HRT ≈ baseline VTE risk; oral HRT ≈ 2–3× baseline risk — this is the single most testable fact from this topic.
- The mechanism is first-pass hepatic metabolism, not the estrogen dose itself.
- VTE risk with oral HRT is highest in the first 1–2 years of use.
- Micronised progesterone/dydrogesterone are the lower-thrombotic-risk progestogen choices.
- For a woman with obesity or thrombophilia wanting HRT: do not withhold HRT — switch the route to transdermal and consider haematology input.
- This guideline itself is archived — if cited in an exam context, current practice references NICE NG23.
Source: RCOG Green-top Guideline No. 19 (Archived) (Published 31 May 2011; archived by RCOG. Current guidance for this topic sits in NICE NG23 (Menopause: diagnosis and management).)
Read the original on rcog.org.uk
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