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Thyroid hormone is essential for normal development, especially of the CNS. In the adult, thyroid hormone maintains metabolic homeostasis and influences the functions of virtually all organ systems. Thyroid hormone contains iodine, which must be supplied by nutritional intake. The thyroid gland contains large stores of thyroid hormone in the form of thyroglobulin. These stores maintain adequate systemic concentrations of thyroid hormone despite significant variations in iodine availability and nutritional intake. The thyroidal secretion is predominantly the prohormone T4, which is converted in the liver and other tissues to supply the plasma with the active form, T3. Local activation of T4 also occurs in target tissues (e.g., brain and pituitary) and is increasingly recognized as an important regulatory step in thyroid hormone action. Similarly, local deactivation of T3 is an important regulatory step. Serum concentrations of thyroid hormones are precisely regulated by the pituitary hormone TSH in a negative-feedback system. The predominant actions of thyroid hormone are mediated via nuclear TRs that modulate the transcription of specific genes.

Overt hyperthyroidism and hypothyroidism, thyroid hormone excess and deficiency, respectively, are associated with numerous clinical manifestations. Milder disease often has a subtler clinical presentation and may be identified based solely on abnormal biochemical tests of thyroid function. Maternal and neonatal hypothyroidism, due to iodine deficiency, remains a major preventable cause of intellectual disability worldwide (Zimmermann, 2009). Treatment of the hypothyroid patient consists of thyroid hormone replacement (Biondi and Wartofsky, 2014). Treatments for hyperthyroidism include antithyroid drugs to decrease hormone synthesis and secretion, destruction of the gland by the administration of radioactive iodine, and surgical removal (Brent, 2008). In most patients, disorders of thyroid function can be either cured or controlled.

Likewise, thyroid malignancies are most often localized and resectable (Haugen et al., 2016; Haugen and Sherman, 2013). Metastatic disease often responds to radioiodine treatment but may become highly aggressive. Radioiodine-refractory, progressive thyroid cancers may respond to targeted chemotherapies, such as tyrosine kinase inhibitors.



CYP: cytochrome P450

Dio1, Dio2, and Dio3: deiodinase types 1, 2, and 3

DIT: diiodotyrosine

ERK: extracellular signal-regulated kinase

GPCR: G protein-coupled receptor

HOI: hypoiodous acid

IGF-1: insulin-like growth factor 1

IP3: inositol 1,4,5-trisphosphate

KISS: potassium iodide (KI) saturated solution

LDL: low-density lipoprotein

MAP kinase: mitogen-activated protein kinase

MCT: monocarboxylic acid transporter

MEK: MAP kinase kinase

MHC: myosin heavy chain, isoform α or β

MIT: monoiodotyrosine

MTC: medullary thyroid carcinoma

NIS: sodium iodide symporter

NO: nitric oxide

NTRK: gene family coding for neutrophic tyrosine receptor kinases (TRKs)

OATP1C1: solute carrier organic anion transporter family, member 1C1

PLC: phospholipase C

RAIU: radioactive iodine uptake

RET: rearranged during transfection tyrosine protein kinase

rT3: reverse T3

T3: 3,5,3′-triiodothyronine

T4: thyroxine

TBG: thyroxine-binding globulin

TR: thyroid hormone receptor

TRH: thyrotropin-releasing hormone


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