Can hyperthyroidism cause intermenstrual bleeding in women of reproductive age?

Can Hyperthyroidism Cause Intermenstrual Bleeding?

Yes, hyperthyroidism can cause menstrual irregularities including intermenstrual bleeding, though oligomenorrhea (decreased menstrual flow) is the most common manifestation, and increased bleeding is relatively rare in hyperthyroid states. 1

Understanding the Menstrual Pattern in Hyperthyroidism

The relationship between hyperthyroidism and menstrual disturbances is well-established but often less severe than historically reported:

• Oligomenorrhea is the predominant pattern, with decreased menstrual flow being more typical than increased bleeding 1
• Only 21.5% of hyperthyroid patients experience menstrual disturbances in modern series, compared to 50% in older literature, likely because hyperthyroidism is now diagnosed earlier when the clinical picture is milder 1, 2
• Anovulatory cycles are very common in hyperthyroidism, which can contribute to irregular bleeding patterns 1
• Increased bleeding (hypermenorrhea, menorrhagia, or metrorrhagia) can occur but is uncommon in hyperthyroid states 1

Clinical Context and Risk Factors

When evaluating a patient with suspected thyroid-related menstrual irregularities:

• Smoking significantly increases the risk of menstrual disturbances in hyperthyroidism: 50% of hyperthyroid patients with irregular cycles were smokers versus only 19% of those with normal cycles 2
• Higher free T4 levels correlate with menstrual abnormalities: patients with irregular periods had mean T4 of 267.7 nmol/L versus 240.6 nmol/L in those with normal cycles 2
• Smokers with hyperthyroidism and irregular periods had even higher T4 levels (280.5 nmol/L) than non-smokers with irregular periods 2

Diagnostic Approach

TSH measurement is warranted as part of the laboratory evaluation of women with abnormal menstrual cycles, though thyroid abnormalities are relatively infrequent causes of menstrual disorders 3

The American College of Clinical Endocrinologists recommends:

• Measure TSH and free T4 to confirm diagnosis and severity of thyroid disease 4
• Obtain a pregnancy test in all reproductive-age women with menstrual irregularities, regardless of reported contraceptive use 4
• Check prolactin levels, as hyperthyroidism can occasionally affect prolactin through various mechanisms 4

Important Clinical Pitfall

Menstrual irregularities may precede other clinical features of thyroid dysfunction by 2 months to 10 years in 45% of cases, meaning reproductive dysfunction may be a presenting symptom before classic thyroid signs appear 5. This underscores the importance of thyroid screening in women with unexplained menstrual changes.

Treatment Implications

Menstrual cycles typically normalize within 2-5 days of achieving adequate thyroid hormone control in hyperthyroidism 4. The primary treatment is addressing the underlying hyperthyroidism rather than treating the bleeding symptom directly, as the menstrual disturbance is secondary to the thyroid dysfunction 6.

For pregnant women with hyperthyroidism:

• Propylthiouracil is preferred in the first trimester, with methimazole preferred in second and third trimesters due to differential teratogenicity risks 6
• Untreated hyperthyroidism increases risks of severe preeclampsia, preterm delivery, heart failure, miscarriage, and low birth weight 6