How does thyroid dysfunction impact menstrual cycles?

Impact of Thyroid Dysfunction on Menstrual Cycles

Thyroid dysfunction significantly impacts menstrual cycles, with both hypothyroidism and hyperthyroidism causing menstrual irregularities in approximately 23-34% of affected women. 1, 2

Types of Menstrual Disturbances by Thyroid Condition

Hypothyroidism

• Primary menstrual abnormalities:

  • Oligomenorrhea (decreased frequency)
  • Menorrhagia/Hypermenorrhea (increased bleeding)
  • Polymenorrhea (increased frequency)
  • Amenorrhea (absent periods) in severe cases 1, 2

• Prevalence:

  • Approximately 23.4% of hypothyroid women experience menstrual disturbances 2
  • Hypermenorrhea is significantly more common in overt hypothyroidism (33%) compared to healthy controls (6%) 3

Hyperthyroidism

• Primary menstrual abnormalities:

  • Oligomenorrhea (most common)
  • Hypomenorrhea (decreased flow)
  • Amenorrhea (less common) 4, 5

• Prevalence:

  • Approximately 21.5% of hyperthyroid women experience menstrual disturbances 5
  • Anovulatory cycles are common despite regular bleeding patterns

Pathophysiological Mechanisms

Hypothyroidism

• Disrupts the hypothalamic-pituitary-gonadal (HPG) axis
• Alters GnRH pulsatility, affecting FSH and LH secretion
• May cause defects in hemostasis leading to increased bleeding 4
• In severe cases, leads to anovulation and reduced fertility 1
• Can affect progesterone production, leading to luteal phase defects

Hyperthyroidism

• Increases sex hormone-binding globulin (SHBG) levels
• Alters estrogen metabolism
• Disrupts the feedback mechanisms of the HPG axis
• Can cause anovulation despite regular-appearing cycles 4

Diagnostic Approach for Menstrual Irregularities

When evaluating menstrual irregularities, thyroid function should be assessed:

1. Laboratory testing:

   • TSH (primary screening test)
   • Free T4 and Free T3 if TSH is abnormal 6
   • Additional hormonal testing: FSH, LH, estradiol, prolactin 6

2. Physical examination:

   • Thyroid examination for enlargement or nodules
   • Signs of hypo/hyperthyroidism
   • BMI calculation 6

Management Considerations

1. For hypothyroidism:

   • Levothyroxine replacement therapy to normalize TSH levels
   • Regular monitoring of thyroid function during treatment
   • Menstrual cycles typically normalize with adequate thyroid replacement 7

2. For hyperthyroidism:

   • Antithyroid medications (propylthiouracil in first trimester, methimazole in second and third trimesters if pregnant) 7
   • Achieve euthyroid state before pregnancy if planning conception 7
   • Monitor for normalization of menstrual patterns

3. Special considerations:

   • Women with thyroid dysfunction should be counseled about potential fertility impacts
   • Thyroid function should be optimized before conception 7
   • Women with unexplained menstrual irregularities should be screened for thyroid dysfunction 6

Clinical Pearls and Pitfalls

• Pearl: The prevalence of menstrual disorders in thyroid dysfunction is lower in recent studies (21-34%) compared to older literature (50-70%), likely due to earlier diagnosis and treatment 5, 2

• Pitfall: Subclinical thyroid dysfunction can also cause menstrual irregularities but may be overlooked if only overt disease is considered

• Pearl: Thyroid dysfunction should be considered in any woman with unexplained menstrual changes, particularly new-onset oligomenorrhea or menorrhagia

• Pitfall: Treating the thyroid disorder alone may not immediately normalize menstrual cycles; additional hormonal management may be temporarily needed in some cases

• Pearl: Both hypothyroidism and hyperthyroidism can reduce fertility and increase risk of adverse pregnancy outcomes, making preconception optimization of thyroid function essential 5, 1