Low T3 and T4 Levels Can Cause Fatigue and Sleepiness
Yes, low levels of triiodothyronine (T3) and thyroxine (T4) can cause fatigue and sleepiness, which are common symptoms of hypothyroidism. 1, 2
Relationship Between Thyroid Hormones and Fatigue
- Untreated hypothyroidism (characterized by low T3 and T4) can lead to fatigue, weight gain, mental slowing, heart failure, and elevated lipid levels 1
- Approximately 80-90% of patients with thyroid dysfunction report complaints about energy levels and fatigue, making it one of the most common symptoms 3
- The "Low T3 Syndrome" has been found at significantly higher rates in patients with chronic fatigue syndrome compared to healthy controls, suggesting a strong relationship between low thyroid hormone levels and fatigue 2
Physiological Mechanisms
- Low T3 and T4 levels reduce cellular metabolism throughout the body, including in the brain and muscles, leading to decreased energy production and resulting fatigue 1
- Patients with low thyroid hormone levels may experience:
- Cold intolerance that doesn't improve with medication
- Dry skin and hair loss
- Constipation
- Depression or mood changes 4
- The body's energy conservation response to low thyroid hormones manifests as fatigue and sleepiness 5
Clinical Considerations
- Both overt hypothyroidism and subclinical hypothyroidism can cause fatigue symptoms 1
- Paradoxically, both hypothyroidism (low T3/T4) and hyperthyroidism can cause fatigue, though through different mechanisms 6
- In patients with normal TSH but persistent fatigue, consider checking free T3 levels, as some patients may have normal T4 but low T3 (Low T3 Syndrome) 2
- Some patients (approximately 15%) continue to experience fatigue and other hypothyroid symptoms even when TSH is normalized with levothyroxine (T4) replacement therapy 7
Diagnostic Approach
- TSH testing is the initial screening test for thyroid dysfunction, with high sensitivity (98%) and specificity (92%) when used to confirm suspected thyroid disease 1
- Additional testing should include free T3 and free T4 levels to assess the actual circulating thyroid hormone levels 6
- Morning cortisol testing may be considered if fatigue is severe, as adrenal issues can coexist with thyroid dysfunction 6
- Be aware that thyroid dysfunction may be masked if the patient is taking beta-blockers 4
Important Clinical Pitfalls
- Patients with both adrenal insufficiency and hypothyroidism should always start steroids prior to thyroid hormone replacement to avoid precipitating an adrenal crisis 4
- Non-thyroidal illness (euthyroid sick syndrome) can also present with low T3 levels and fatigue, especially during caloric deprivation or severe illness 5
- When treating hypothyroidism, monitoring both T3 and T4 levels is important, not just TSH, as some patients may have normal TSH but low T3 levels 8
- Consider endocrinology referral for complex cases or when standard therapy fails to resolve fatigue symptoms 4