Uses of free T4, T3, and imaging
If TSH is elevated, a free T4 level can help differentiate between central hyperthyroidism and much more common peripheral hypothyroidism (99% of patients). A T3 level is necessary only if the TSH is undetectable and the free T4 is normal.7,8 Imaging the thyroid gland is reserved for evaluating structural abnormalities.
When to reassess TSH
In primary hypothyroidism, reassess TSH 6 weeks after the start of treatment or a change in replacement dose.4 This recommendation is based on the fact that levothyroxine has a half-life of about a week; a steady state would be achieved over the course of 5 half-lives. No direct patient-oriented evidence exists for this testing interval. Test free T4 if you suspect excessive replacement or noncompliance.
Once TSH is in the normal range, experts recommend assessing the level after 6 months and then annually.4 A 2005 study suggested lowering the target TSH level for assessing adequate replacement in patients treated with standard levothyroxine because subtherapeutic T3 levels were found despite normal TSH levels in these patients.9
TSH can’t be used to monitor therapy for central hypothyroidism. Follow both free T4 and T3 concentrations because elevated T3 levels, indicative of overtreatment, can occur even when free T4 measurements are normal in these patients.10
Recommendations
ACP Medicine recommends TSH as the initial test. If TSH is elevated, ACP Medicine advises confirmation with a repeat TSH plus a free T4.11