Heavy Menstrual Bleeding
Thyroid hormone influences the menstrual cycle, and an underactive thyroid is a recognised cause of heavy or prolonged periods. An overactive thyroid can cause lighter or irregular bleeding.
Because heavy periods have many causes, thyroid testing is one part of a broader assessment.
Last medically reviewed: 15 November 2025
Key points
- Hypothyroidism can cause heavy, prolonged or irregular periods.
- Hyperthyroidism can cause lighter, shorter or absent periods.
- Other common causes include fibroids, hormonal imbalance and bleeding disorders.
- Treating thyroid dysfunction often improves cycle regularity and flow.
How the thyroid affects periods
Thyroid hormone interacts with the hormones that control ovulation and the uterine lining. When thyroid levels are low, sex hormone binding globulin and other regulators change, which can lead to heavier, longer or irregular bleeding.
In an overactive thyroid, periods may become lighter, shorter or stop altogether.
Other causes to consider
Heavy menstrual bleeding is common and often due to fibroids, adenomyosis, polyps, hormonal imbalance, bleeding disorders or medication. Thyroid disease is one contributor among many.
A clinician will assess the pattern, associated symptoms and, where needed, arrange ultrasound or blood tests.
Investigation
Assessment includes a menstrual history, examination, thyroid function tests and often a full blood count to check for iron-deficiency anaemia. Pelvic ultrasound may be arranged.
If thyroid disease is found, it is treated and the cycle is reviewed afterwards.
Management
Correcting thyroid dysfunction often improves flow and regularity, though this can take several cycles. Iron deficiency should be treated. Persistent heavy bleeding needs gynaecological assessment regardless of thyroid status.
When to seek urgent medical help
- Bleeding so heavy that you feel faint, breathless or exhausted.
- Bleeding between periods or after intercourse.
- Bleeding after menopause.
This guidance does not replace emergency services. If you are seriously unwell, seek urgent medical care immediately.
Frequently asked questions
Can thyroid problems cause heavy periods?
Yes — hypothyroidism is a recognised cause of heavy, prolonged or irregular periods, and treating it often improves the cycle.
Can an overactive thyroid affect periods?
Yes — it can make periods lighter, shorter or absent.
Will my periods return to normal after treatment?
Often they improve within a few cycles, though this varies and other causes may also need attention.
Do I need a pelvic ultrasound?
Possibly — heavy bleeding has many causes, and ultrasound is commonly used to check for fibroids, polyps or other structural problems.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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