Lexicon
Thyroid Function
Also known as TSH
Definition
Hypothyroidism is the common clinical condition of thyroid hormone deficiency which, if left untreated, can lead to serious adverse health effects on multiple organ systems. [1] Overt primary hypothyroidism is defined as an elevated thyroid-stimulating hormone concentration in combination with a free thyroxine concentration below the reference range. [1] Subclinical hypothyroidism, commonly considered an early sign of thyroid failure, is defined by an elevated TSH concentration with free thyroxine within the reference range. [1]
How it works
Hypothyroidism is classified as primary, central, or peripheral based on pathology in the thyroid, in the pituitary or hypothalamus, or in peripheral tissue, respectively. [1] Because the pituitary raises TSH output in response to falling thyroid hormone, an elevated TSH with a low free thyroxine indicates primary thyroid failure. [1] Genetic factors contribute up to 65% of the interindividual variation in TSH and thyroid hormone levels, and environmental factors such as smoking, body mass index, and iodine intake also affect them. [2] Thyroid hormones are key determinants of cellular metabolism, regulating carbohydrate, lipid and protein pathways as well as mitochondrial respiration and biogenesis across target tissues [3].
Connected concepts
How it's measured
TSH and free thyroid hormone tests are frequently used to assess the functional status of the thyroid, alongside antibody and tumour-marker tests for specific diagnoses. [4] The diagnosis of subclinical hypothyroidism should be confirmed by repeat thyroid function tests, ideally obtained at least 2 months later, because 62% of elevated TSH levels may revert to normal spontaneously. [5] Automated TSH immunoassays are vulnerable to interferences such as biotin and heterophilic antibodies, so discordant results should prompt suspicion of assay artefact before further investigation or treatment. [6]
What levels mean
Serum thyrotropin concentrations rise as people without thyroid disease age, and may surpass the traditional reference range of 4 to 5 mU/L in elderly patients. [7] In older individuals, levothyroxine should be considered for subclinical hypothyroidism when TSH is persistently 7 mIU/L or higher, and not initiated when TSH is less than 7 mIU/L. [8] Treatment might be indicated for patients with subclinical hypothyroidism and serum thyrotropin levels of 10 mU/L or higher, or for young and middle-aged individuals with symptoms consistent with mild hypothyroidism.
Community knowledge
## Laboratory Assessment
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- 1.Chaker L, Razvi S, Bensenor IM, Azizi F, Pearce EN, Peeters RP. Hypothyroidism. Nat Rev Dis Primers · 2022
- 2.Babić Leko M, Gunjača I, Pleić N, Zemunik T. Environmental Factors Affecting Thyroid-Stimulating Hormone and Thyroid Hormone Levels. Int J Mol Sci · 2021
- 3.Cicatiello AG, Di Girolamo D, Dentice M. Metabolic Effects of the Intracellular Regulation of Thyroid Hormone: Old Players, New Concepts. Front Endocrinol (Lausanne) · 2018
- 4.Soh SB, Aw TC. Laboratory Testing in Thyroid Conditions - Pitfalls and Clinical Utility. Ann Lab Med · 2019
- 5.Ross DS. Treating hypothyroidism is not always easy: When to treat subclinical hypothyroidism, TSH goals in the elderly, and alternatives to levothyroxine monotherapy. J Intern Med · 2022
- 6.Favresse J, Burlacu MC, Maiter D, Gruson D.
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