Recently, a trial with 353 subjects on steady-state L-T4 replacement dose for autoimmune thyroiditis or after surgery for malignant or benign thyroid disease was published in Endocrine Connections.

It revealed that substitution dose was independently associated with gender, age, aetiology and deiodinase capacity (all p<0.001). Stratifying patients by deiodinase activity (SPINA-GD) categories of <23, 23-29 and >29 nmol/s revealed an increasing FT3-FT4 dissociation; the poorest converters showed the lowest FT3 levels in spite of highest dose and circulating FT4 (p<0.001).

As the authors state these findings imply that a) thyroid hormone conversion efficiency is an important modulator of the biochemical response to levothyroxine, b) measuring FT3 may be an additional treatment target, and c) L-T4 dose escalation may have limited success to raise FT3 appropriately in some cases.

The full text of the paper is available for free from http://dx.doi.org/10.1530%2FEC-15-0056 .

Reference
Midgley JE, Larisch R, Dietrich JW, Hoermann R. Variation in the Biochemical Response to L-Thyroxine Therapy and Relationship with Peripheral Thyroid Hormone Conversion. Endocr Connect. 2015 Aug 11. pii: EC-15-0056. [Epub ahead of print] PubMed PMID: 26265111