Am I unfair if I say that this paper appears to be absolure support for FT3 testing - within the reference range for TSH?
Really needs to be extended to those with low or suppressed TSH to make a proper picture.
J Clin Endocrinol Metab. 2017 Apr 28. doi: 10.1210/jc.2017-00224. [Epub ahead of print]
Thyroid function variation in the normal range, energy expenditure and body composition in L-T4 treated subjects.
Samuels MH1, Kolobova I2, Antosik M3, Niederhausen M4, Purnell JQ5, Schuff KG6.
1 Division of Endocrinology, Diabetes and Clinical Nutrition Oregon Health & Science University 3181 SW Sam Jackson Park Road Portland, OR 97239 firstname.lastname@example.org.
2 Behavioral Science Director Penn State Health St. Joseph Family Medicine Residency 145 N. 6th St., Suite 2000 Reading, PA 19603 email@example.com.
3 Legacy Emanuel Medical Center 2801 N Gantenbein Ave Portland, OR 97227 firstname.lastname@example.org.
4 Biostatistics & Design Program OHSU-PSU School of Public Health 3181 SW Sam Jackson Park Road Portland, OR 97239 email@example.com.
5 Knight Cardiovascular Institute Division of Endocrinology, Diabetes and Clinical Nutrition Oregon Health & Science University 3181 SW Sam Jackson Park Road Portland, OR 97239 firstname.lastname@example.org.
6 Division of Endocrinology, Diabetes and Clinical Nutrition Oregon Health & Science University 3181 SW Sam Jackson Park Road Portland, OR 97239 email@example.com.
It is not clear whether upper limits of the 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.
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 (0.34-5.6 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.
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 T3 levels were directly correlated with resting energy expenditure, BMI, body fat mass, and visceral fat mass, with clinically relevant variations in these outcomes.
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.