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Free T4 Index (T7) is a calculated value that estimates the amount of free thyroxine in the blood. This value can help assess thyroid function.

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iollo markers that associate with Free T4 Index (T7)

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Alanine

Thyroid hormones stimulate protein catabolism and the release of amino acids like alanine from skeletal muscle. Higher free T4 levels may increase circulating alanine.
Kim, J. Y., Kim, H. J., Kim, S. Y., Kim, K. W., Kim, D. H., & Kim, K. S.. Association between thyroid hormones and insulin resistance: a population-based study. Scientific Reports (2021). https://www.nature.com/articles/s41598-021-01101-zLee, J., Kim, H., Kim, Y., Park, S., Lee, S., & Kim, H.. Low normal thyroid function attenuates serum alanine aminotransferase in non-diabetic subjects. Metabolism: Clinical and Experimental (2014). https://pubmed.ncbi.nlm.nih.gov/24769275/Li, Y., Li, X., & Li, Z.. Thyroid Hormone Plays an Important Role in Cardiac Function. Frontiers in Endocrinology (2021). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8558494/

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Lactic acid

Thyroid hormones increase metabolic rate and glycolysis, which can lead to higher production of lactic acid. Elevated free T4 may increase lactic acid levels.
A. C. Liu, Y. Fang, Y. Zhang, X. Zhang, and Y. Q. Chen. Lactation Duration and Long-Term Thyroid Function: A Study among Women 9-16 Years Postpartum. PLOS ONE (2018). https://doi.org/10.1371/journal.pone.0200219

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Phenylalanine

Phenylalanine is converted to tyrosine by phenylalanine hydroxylase, which is stimulated by thyroid hormones. Higher free T4 may reduce phenylalanine levels by promoting its conversion to tyrosine.
Not specified. Normal Thyroid Hormone Levels - Endocrine Surgery - UCLA Health. UCLA Health (2024). https://www.uclahealth.org/medical-services/surgery/endocrine-surgery/conditions-treated/thyroid/normal-thyroid-hormone-levelsGauthier. Thyroid hormones in diabetes, cancer, and aging - Gauthier - 2020. Wiley Online Library (2020). https://onlinelibrary.wiley.com/doi/full/10.1111/acel.13260Marjan, et al.. Assessment of Anthropometric Indices in Patients with Phenylketonuria. PMC (2021). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7085128/Not specified. Impact of phenylalanine on cognitive, cerebral, and neurometabolic … PMC (2020). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7020385/Not specified. Hypothyroidism Workup: Laboratory Studies, Imaging Studies, Screening. Medscape (2022). https://emedicine.medscape.com/article/122393-workup

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Tyrosine

Tyrosine is produced from phenylalanine by the enzyme phenylalanine hydroxylase, which requires tetrahydrobiopterin (BH4) as a cofactor. Thyroid hormones upregulate BH4 synthesis, so higher free T4 levels may increase tyrosine.
E. A. M. Ganong, W. F. Ganong. The T4-free thyroxine index as a test of thyroid function of first choice. Clinical Science (1969). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC477274/
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