Thyroid Hormone Homeostasis in Levothyroxine-treated Patients: Findings From ELSA-Brasil.

Initiation of LT4 therapy resulted in decreased TSH, increased FT4, and reduced FT3 serum levels.

• TSH decreased by 11% to 19% following initiation of LT4 therapy
• FT4 increased by approximately 19% following initiation of LT4 therapy
• FT3 serum levels were reduced by 7% following initiation of LT4 therapy
• FT3 dropped more than 10% in approximately 40% of LT4-treated patients
• Analysis was based on 186 participants who initiated LT4 treatment during the study

Participants continuously treated with LT4 exhibited stable elevation in serum FT4 and reductions in serum FT3 and TSH levels.

• 243 individuals were continuously treated with LT4 therapy throughout the study
• Continuously treated participants showed a stable elevation in serum FT4
• Continuously treated participants showed a reduction in both serum FT3 and TSH levels
• The ELSA-Brasil study followed participants over 8.2 years (2008-2019) with 3 observation points

A substantial proportion of continuously LT4-treated patients had serum FT4 levels above the normal reference range.

• 115 participants (47.3%) had at least 1 serum FT4 level above the control reference range
• The upper limit of the control reference range for FT4 was defined as greater than 1.52 ng/dL
• These elevated FT4 levels occurred despite normal TSH levels

A notable proportion of continuously LT4-treated patients had serum FT3 levels below the normal reference range.

• 38 participants (15.6%) had at least 1 serum FT3 below the reference range
• The lower limit of the control reference range for FT3 was defined as less than 0.23 ng/dL
• These low FT3 levels occurred despite normal TSH levels

Initiation of LT4 therapy was associated with changes in metabolic parameters and medication utilization.

• LT4 initiation was associated with an increase in triglyceride levels
• LT4 initiation was associated with increased utilization of hypolipidemic medications
• LT4 initiation was associated with increased utilization of antidiabetic medications

The study population consisted of adult Brazilians followed longitudinally with thyroid hormone measurements at multiple time points.

• The ELSA-Brasil study follows 15,105 adult Brazilians aged 35-74 years
• The study period was 8.2 years from 2008 to 2019 with 3 observation points
• Serum parameters assessed included TSH, free T4 (FT4), and free T3 (FT3) levels
• 186 participants initiated LT4 treatment during the study and 243 were continuously treated with LT4