The Use of Levothyroxine Absorption Tests in Clinical Practice.

Area of Science:

• Endocrinology
• Pharmacology
• Clinical Diagnostics

Background:

• Levothyroxine (LT4) is a common treatment for hypothyroidism.
• Over 30% of patients on LT4 do not achieve target thyrotropin levels.
• LT4 absorption testing (LT4AT) is used to assess absorption and diagnose malabsorption.

Purpose of the Study:

• To review and analyze existing levothyroxine absorption test (LT4AT) procedures.
• To propose a standardized protocol for routine LT4AT.
• To establish criteria for diagnosing normal or abnormal LT4 absorption.

Main Methods:

• Systematic literature search of PubMed using relevant keywords.
• Analysis of 43 full-text publications on LT4AT procedures.
• Proposal of a standardized protocol including fasting, a 1000 µg LT4 dose, supervised 4-hour testing with hourly FT4 sampling.

Main Results:

• Published LT4AT procedures show significant variability in dose, formulation, duration, sampling, and interpretation.
• A proposed standardized LT4AT involves a 1000 µg test dose, 4-hour supervised monitoring, and hourly FT4 measurements.
• Normal absorption criteria include an FT4 increment >0.40 ng/dL or TT4 increment >6 μg/dL (with specific dose considerations).

Conclusions:

• Standardization of the LT4AT is needed for reliable diagnosis of malabsorption.
• The proposed protocol provides a framework for consistent LT4 absorption assessment.
• Abnormal results guide management, including dose adjustment, formulation changes, or specialist referral.