Only FT3 tracks symptom improvement during treatment.
This is a fairly recent and terrific piece of research that clearly places FT3 centre place, as the most useful lab test measure, as it correlates to symptoms more than the other thyroid laboratory test results.
Moreover, the research shows that in T4-monotherapy many patients did not find symptomatic relief until FT3 was elevated in the range, and TSH was suppressed.
The research findings are especially relevant to those patients on either T4 monotherapy or a T3/T4 combination who are still struggling with some hypothyroid symptoms.
Clearly, other issues should be ruled out (cortisol, iron etc.). However, when symptoms remain it is always very important to focus on the symptoms themselves, and the signs (like body temperature) and the FT3 level plus reverse T3.
FT3 is the most important thyroid lab level, as only changes in FT3 correlate to symptom changes in patients. If FT3 stays the same and FT4 goes up, this is unlikely to result in symptom improvement. Likewise, if FT3 goes up but FT4 and reverse T3 also go up, this may also be unlikely to result in symptom improvement.
We usually want to see better levels of FT3 without any significant increase in reverse T3. Changes in FT4 level are far less significant. Even having FT4 at the bottom of the lab range or lower is unimportant, as long as FT3 is at a good level and the patient’s symptoms are improving.
Here is the research article:
“Symptomatic Relief is Related to Serum Free Triiodothyronine Concentrations during Follow-up in Levothyroxine-Treated Patients with Differentiated Thyroid Cancer”
Rolf Larisch, John E M Midgley, Johannes W Dietrich, Rudolf Hoermann
Exp Clin Endocrinol Diabetes 2018; 126(09): 546-552
All the information in this blog post is contained within my ‘The Thyroid Patient’s Manual’ book. The book is a ‘manual’ that includes many critical pieces of information that all thyroid patients require. It was written based on my thirty-plus years of researching thyroid disease, and the best possible approaches to dealing with hypothyroidism, based on the latest research.