Hormone Therapy and Fatigue: Causes, What to Expect, and How to Cope

Hormone therapy can be associated with fatigue, and this can range from mild tiredness to a more intense, persistent lack of energy. Many people on breast cancer hormone therapies (like aromatase inhibitors or tamoxifen) and other cancer treatments report fatigue as a common side effect. ^[1] Fatigue can also occur with radiation therapy and immunotherapy and may persist for weeks to months after treatment ends. ^{[2] [3]} Changes in hormone levels themselves can contribute to feeling unusually tired or weak. ^[4]

Why Hormone Therapy Can Cause Fatigue

• Breast cancer hormone therapy: Treatments such as aromatase inhibitors often list fatigue among common side effects, along with hot flashes and joint pain. ^[1] Tamoxifen can also cause fatigue, among other effects. ^[1]
• Radiation-related fatigue overlap: If hormone therapy is given alongside radiation, you may notice added tiredness starting 2–3 weeks into treatment, sometimes lasting months after completing therapy. ^{[2] [3]}
• Immunotherapy interactions: Some people receiving immunotherapy experience fatigue due to thyroid changes; care teams often check thyroid function if fatigue worsens. ^[5]
• General cancer-related fatigue: Shifts in hormones, sleep issues, stress, and the body’s response to treatment all contribute to low energy that isn’t fully relieved by rest. This type of fatigue can be more intense and linger. ^[4]

What Fatigue Feels Like

Fatigue can feel like heaviness in arms and legs, trouble focusing, irritability, or feeling “slowed down.” It can start suddenly or build gradually and may not improve with sleep alone. ^[6]

Practical Coping Strategies

Daily Energy Management

• Plan short rest breaks: Take 15–20 minute naps rather than long daytime sleeps; shorter naps can help fatigue without disturbing nighttime sleep. ^[7]
• Conserve energy: Prioritize essential tasks, pace yourself, and ask for help with chores or transportation when needed. ^[8]
• Gentle exercise: Light activity, such as walking, can help raise energy and reduce fatigue; people who stay lightly active often handle treatment better. ^[7]
• Healthy nutrition: Eat small, balanced meals with adequate protein; hydration (about 8–10 cups of liquids a day) supports energy. ^{[9] [10]}
• Sleep routine: Keep a consistent bedtime, limit late caffeine, and create a restful environment; this complements short daytime rests. ^[6]

When to Talk to Your Care Team

• Persistent or worsening fatigue: If fatigue doesn’t improve with rest or limits daily activities, let your clinician know so they can look for treatable causes (like anemia, thyroid issues, pain, depression, or medication effects). ^[4]
• Combined therapies: If you are receiving radiation or immunotherapy along with hormone therapy, inform your team about fatigue patterns; they may adjust schedules or check labs (for example, thyroid levels). ^{[2] [5]}

Specific Considerations by Treatment Type

• Aromatase inhibitors and tamoxifen (breast cancer): Expect possible fatigue among other symptoms; simple steps like light exercise, hydration, and pacing often help. Report severe or new fatigue, as adjustments or supportive care may be appropriate. ^{[1] [11]}
• Radiation therapy: Recognize that fatigue commonly starts after a few weeks and can continue for months; consistent gentle movement and structured rest are key. ^{[2] [3]}
• Immunotherapy: Because fatigue can be tied to thyroid changes, routine monitoring and timely treatment of thyroid dysfunction can improve energy. ^[5]
• General cancer care: Treatable contributors to fatigue should be identified and managed; don’t assume fatigue is “just part of treatment.” Your care team can offer tailored interventions. ^{[4] [12]}

Safety Tips

• Avoid new supplements without medical advice: Some vitamins or stimulants can interfere with therapies; always check with your clinician before starting anything new. ^[13]
• Track patterns: Note times of day when fatigue is worst (often midafternoon) and plan rests and activities accordingly. A simple log helps you and your team spot trends. ^[8]
• Seek support: Social workers and community organizations can assist with practical tasks, reducing energy drain. ^[9]

Bottom Line

Fatigue can be a common side effect of hormone therapy and other cancer treatments, often driven by changes in hormones and the body’s response to therapy. With pacing, short naps, gentle activity, good nutrition, and timely medical evaluation for treatable causes, many people find fatigue becomes more manageable. ^{[1] [7] [4] [6]}