Thymus Involution across the Human Menstrual Cycle

Area of Science:

• Immunology
• Endocrinology
• Human Physiology

Background:

• Thymic involution is traditionally viewed as a linear, age-related process.
• Animal studies indicate hormone-dependent thymus size fluctuations during reproductive cycles.
• Translational evidence in humans regarding menstrual cycle-related thymus dynamics is limited.

Purpose of the Study:

• To investigate dynamic changes in thymus size across the human menstrual cycle.
• To assess the association between thymus size variations and fluctuations in estradiol and progesterone levels.
• To determine the influence of hormonal contraception on menstrual cycle-related thymus size changes.

Main Methods:

• An observational, repeated-measures study utilizing quantitative computed tomography (qCT).
• Assessments were conducted during menses and the early luteal phase within the same menstrual cycle.
• Mixed-effects models analyzed associations between cycle phase, hormone levels, hormonal contraceptive status, and thymus size.

Main Results:

• Estradiol levels increased from menses to the early luteal phase, irrespective of hormonal contraceptive use.
• Thymus size differed significantly between hormonal contraceptive users and non-users, with larger volumes in non-users.
• Thymic involution from menses to the early luteal phase was observed in 66.7% of non-users versus 37.5% of users; estradiol and progesterone interacted in predicting thymus size.

Conclusions:

• This study provides the first human evidence of short-term, dynamic changes in thymus involution across the menstrual cycle.
• Reproductive hormones (estradiol, progesterone) and hormonal contraception influence these thymus size fluctuations.
• Findings suggest potential implications for sex differences in immune regulation and inflammatory diseases.