Should You Check Free T4 in This Patient?
Yes, you should check a free T4 level in this patient. A TSH of 4.64 mIU/L in a patient taking levothyroxine 88 µg daily indicates inadequate thyroid hormone replacement, and measuring free T4 will help distinguish between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4), which directly impacts your management approach 1.
Why Free T4 Measurement Is Essential Here
The free T4 result determines your next clinical decision. When TSH is elevated in a patient already on levothyroxine, you need to know whether the patient has:
- Overt hypothyroidism (low free T4): Requires immediate dose increase and more aggressive titration 1
- Subclinical hypothyroidism (normal free T4): Still warrants dose adjustment given TSH >4.5 mIU/L, but with different urgency and monitoring 1
Free T4 helps interpret ongoing abnormal TSH levels during therapy, as TSH may take longer to normalize than free T4 1. This is particularly relevant in your patient who is already on treatment but not achieving target TSH.
Clinical Context Supporting Free T4 Measurement
Your patient's TSH of 4.64 mIU/L clearly indicates undertreatment. The target TSH range for patients on levothyroxine is 0.5–4.5 mIU/L 1. At 60 years old taking 88 µg daily, this patient should have normalized TSH by now if the dose were adequate 1.
Measuring both TSH and free T4 distinguishes between subclinical hypothyroidism (normal free T4) and overt hypothyroidism (low free T4) 1. This distinction matters because:
- If free T4 is low, the patient has overt hypothyroidism requiring more substantial dose increases (25 µg increments) 1
- If free T4 is normal, the patient has subclinical hypothyroidism, and you can use smaller increments (12.5–25 µg) 1
What the Free T4 Result Will Tell You
The free T4 level guides your dose adjustment strategy:
- Low free T4: Increase levothyroxine by 25 µg to 113 µg daily, as this represents overt hypothyroidism 1
- Normal free T4: Increase by 12.5–25 µg (to 100–113 µg daily), as this represents subclinical hypothyroidism with TSH in the 4.5–10 mIU/L range 1
After any dose adjustment, recheck TSH and free T4 in 6–8 weeks 1. The free T4 measurement at that time will confirm whether the dose increase was sufficient or if further titration is needed 1.
Common Pitfalls to Avoid
Do not adjust the levothyroxine dose based on TSH alone without knowing the free T4 level. Approximately 25% of patients on levothyroxine are unintentionally maintained on inappropriate doses, and measuring free T4 helps prevent both undertreatment and overtreatment 1.
Do not assume the patient is non-adherent without biochemical confirmation. While non-adherence is common, measuring free T4 helps distinguish true inadequate dosing from pseudomalabsorption or non-adherence 2. If free T4 is surprisingly normal despite elevated TSH, consider adherence issues or malabsorption 2.
Do not wait to see if TSH normalizes on its own. TSH of 4.64 mIU/L in a patient already on levothyroxine for hypothyroidism indicates the current dose is insufficient and requires adjustment 1. Even for subclinical hypothyroidism with TSH between 4.5–10 mIU/L, treatment is reasonable when the patient is already on thyroid replacement therapy 1.