TSH 0.40: Should You Check T3 or T4?
Check free T4 first—it's the essential next step when TSH is at the lower end of normal to distinguish between normal thyroid function, subclinical hyperthyroidism, and overtreatment if you're on levothyroxine. 1, 2
Why Free T4 is the Priority
TSH alone cannot determine thyroid status. A TSH of 0.40 mIU/L sits at the lower boundary of the normal reference range (typically 0.4-4.5 mIU/L), making it impossible to distinguish between normal variation, early subclinical hyperthyroidism, or appropriate thyroid function without measuring free T4. 1, 3
Free T4 distinguishes critical clinical scenarios. If your free T4 is normal, you're euthyroid and require no intervention. If free T4 is elevated with TSH 0.40, this indicates overt hyperthyroidism requiring prompt treatment. If you're on levothyroxine and free T4 is high-normal to elevated, this suggests overtreatment. 2, 3
Low TSH with normal T4 is common in older adults and doesn't indicate disease. Studies show that approximately 3.9% of persons over 60 have TSH <0.1 mIU/L, yet about half are euthyroid with normal T4 levels and never develop hyperthyroidism during years of follow-up. 3
What About T3?
T3 is not the initial test of choice for TSH 0.40. Free T4 provides more diagnostic value than T3 for distinguishing thyroid dysfunction, as T4 shows a stronger inverse relationship with TSH (r=0.73) compared to T3 (r=0.41). 4
T3 becomes relevant only after confirming hyperthyroidism. If your free T4 is elevated, then measuring T3 helps establish the etiology—a T3/T4 ratio >18.9 suggests Graves' disease or toxic multinodular goiter, while <16 suggests thyroiditis. 5
T3 alone can be misleadingly normal. Even in hypothyroidism, T3 levels may remain normal or elevated while T4 is low, demonstrating that T3 doesn't reliably reflect overall thyroid status. 4
Clinical Algorithm
If you're NOT on levothyroxine:
- Measure free T4 now 1, 3
- If free T4 is normal (typically 9-19 pmol/L or 0.7-1.5 ng/dL): You're euthyroid—no further testing needed unless symptoms develop 1
- If free T4 is elevated: Confirm with repeat testing in 2-4 weeks, then measure T3 and proceed with radioactive iodine uptake scan to determine etiology 2
- If free T4 is low: This suggests central hypothyroidism (pituitary/hypothalamic dysfunction)—requires endocrine evaluation 1
If you ARE on levothyroxine:
- Measure free T4 immediately 1
- TSH 0.40 with elevated free T4 indicates overtreatment—reduce dose by 12.5-25 mcg 1
- TSH 0.40 with normal free T4 is appropriate replacement—no dose change needed 1
- Recheck TSH and free T4 in 6-8 weeks after any dose adjustment 1
Critical Pitfalls to Avoid
Don't assume TSH 0.40 means hyperthyroidism. This value is within the normal reference range for most laboratories, and low-normal TSH without elevated T4 does not indicate disease requiring treatment. 1, 3
Don't order T3 as your first test. The positive predictive value of low TSH alone for hyperthyroidism is only 12% in older adults, but adding T4 measurement increases this to 67%. T3 adds no diagnostic value at this stage. 3
Don't treat based on a single borderline value. TSH varies due to pulsatile secretion, time of day, acute illness, and medications. Confirm any abnormality with repeat testing before making treatment decisions. 1
Don't ignore clinical context. If you have cardiac disease, atrial fibrillation, or are elderly, even subclinical hyperthyroidism (TSH <0.1 with normal T4) carries increased risk for arrhythmias and fractures—but TSH 0.40 doesn't meet this threshold. 1, 2
When to Recheck
If asymptomatic with normal free T4: No routine follow-up needed unless symptoms develop (palpitations, tremor, heat intolerance, weight loss, anxiety). 1
If on levothyroxine with appropriate dosing: Recheck TSH and free T4 in 6-12 months or if symptoms change. 1
If free T4 is abnormal: Repeat testing in 2-4 weeks to confirm, and expedite to 2 weeks if you have cardiac symptoms or serious medical conditions. 2