Thyroid function variation in the normal range, energy expenditure, and body composition in l-t4-treated subjects

Mary Samuels, Irina Kolobova, Megan Antosik, Meike Niederhausen, Jonathan Purnell, Kathryn Schuff

Research output: Contribution to journalArticle

11 Citations (Scopus)

Abstract

Purpose: It is not clear whether upper limits of the thyrotropin (TSH) reference range should be lowered. This debate can be better informed by investigation of whether variations in thyroid function within the reference range have clinical effects. Thyroid hormone plays a critical role in determining energy expenditure, body mass, and body composition, and therefore clinically relevant variations in these parameters may occur across the normal range of thyroid function. Methods: This was a cross-sectional study of 140 otherwise healthy hypothyroid subjects receiving chronic replacement therapy with levothyroxine (L-T4) who had TSH levels across the full span of the laboratory reference range (0.34 to 5.6 mU/L). Subjects underwent detailed tests of energy expenditure (total and resting energy expenditure, thermic effect of food, physical activity energy expenditure), substrate oxidation, diet intake, and body composition. Results: Subjects with low-normal (#2.5 mU/L) and high-normal (.2.5 mU/L) TSH levels did not differ in any of the outcome measures. However, across the entire group, serum free triiodothyronine (fT3) levels were directly correlated with resting energy expenditure, body mass index (BMI), body fat mass, and visceral fat mass, with clinically relevant variations in these outcomes. Conclusions: Variations in thyroid function within the laboratory reference range have clinically relevant correlations with resting energy expenditure, BMI, and body composition in L-T4-treated subjects. However, salutary effects of higher fT3 levels on energy expenditure may be counteracted by deleterious effects on body weight and composition. Further studies are needed before these outcomes should be used as a basis for altering L-T4 doses in L-T4-treated subjects.

Original languageEnglish (US)
Pages (from-to)2533-2542
Number of pages10
JournalJournal of Clinical Endocrinology and Metabolism
Volume102
Issue number7
DOIs
StatePublished - Jul 1 2017

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Body Composition
Energy Metabolism
Thyroid Gland
Reference Values
Chemical analysis
Body Mass Index
Fats
Intra-Abdominal Fat
Triiodothyronine
Thyrotropin
Nutrition
Thyroxine
Thyroid Hormones
Adipose Tissue
Healthy Volunteers
Cross-Sectional Studies
Hot Temperature
Body Weight
Outcome Assessment (Health Care)
Exercise

ASJC Scopus subject areas

  • Endocrinology, Diabetes and Metabolism
  • Biochemistry
  • Endocrinology
  • Clinical Biochemistry
  • Biochemistry, medical

Cite this

Thyroid function variation in the normal range, energy expenditure, and body composition in l-t4-treated subjects. / Samuels, Mary; Kolobova, Irina; Antosik, Megan; Niederhausen, Meike; Purnell, Jonathan; Schuff, Kathryn.

In: Journal of Clinical Endocrinology and Metabolism, Vol. 102, No. 7, 01.07.2017, p. 2533-2542.

Research output: Contribution to journalArticle

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