Endocrine Abstracts

Background: Since serum free thyroxine levels raise after levothyroxine ingestion and take several hours before returning to baseline, thyrotropin is recommended as a laboratory test for monitoring the adequacy of levothyroxine replacement in primary hypothyroidism patients. However, immunoglobulin-bound thyrotropin causing falsely high thyrotropin levels is commonly found and leads to unnecessary increase in levothyroxine dosage. Polyethylene glycol (PEG) precipitation is an accepted method for correcting this interference.

Objective: To use serum thyrotropin in combination with free thyroxine levels obtained before levothyroxine dosing in order to reduce unnecessary adjustment of levothyroxine dosage due to immunoglobulin-bound thyrotropin in patients with primary hypothyroidism receiving levothyroxine.

Methods: This is a cross-sectional study using blood specimens obtained before levothyroxine dosing from the patients with primary hypothyroidism receiving levothyroxine. Thyrotropin and free thyroxine were measured by electrochemiluminescence immunoassay. A 25% PEG-6000 solution was used to precipitate immunoglobulin-bound thyrotropin, and thyrotropin levels after PEG precipitation were used as the reference range.

Results: In 125 patients, 60 patients had normal thyrotropin levels (0.27-4.2 uIU/mL) and 65 patients had elevated thyrotropin levels (>4.2 uIU/mL). In patients with normal thyrotropin levels, 51 (85%) had free thyroxine levels ≥1.2 ng/dL, and all had thyrotropin levels after PEG precipitation within normal range. In those with elevated thyrotropin levels, 51 (78.5%) had free thyroxin <1.2 ng/dL and 14 (21.5%) had free thyroxine ≥1.2 ng/dl. In patients with free thyroxin <1.2 ng/dL, 37 (72.5%) still had elevated thyrotropin levels after PEG precipitation, while the rest had normal levels. In patients with free thyroxin ≥1.2 ng/dL, all but 1 (93%) had normal thyrotropin levels after PEG precipitation.

Conclusion: It is unnecessary to increase levothyroxine dosage in patients with primary hypothyroidism who have elevated thyrotropin levels if free thyroxine levels before levothyroxine dosing are 1.2 ng/dL or more.