The thyroid gland regulates metabolism and affects almost every cell in the body. The primary hormone produced by the thyroid is thyroxine (T4), a minimally active form that must be converted to T3 to exert its actions. Measurement of total T4 includes the majority, which is bound to protein, and the free form, which is a better indicator of thyroid status. Low T4 is associated with overt hypothyroidism, iodine insufficiency, decreased energy expenditure, weight gain, and increased cholesterol. Elevated T4 is associated with overt hyperthyroidism, thyroid cancer, increased energy expenditure, reduced cholesterol, and pregnancy.
Standard Range: 4.50 – 12.00 ug/dL (57.92 – 154.44 nmol/L)
The ODX Range: 6.00 – 11.90 ug/dL (77.22 – 153.15 nmol/L)
Low total T4 is associated with hypothyroidism, Hashimoto thyroiditis, iodine insufficiency, thyroidectomy, thyroid agenesis, pituitary insufficiency, hypothalamic failure, myxedema, cirrhosis, renal failure, Cushing syndrome, TSH receptor defects, certain medications including anabolic steroids, androgens, lithium, propranolol, phenytoin, and anti-thyroid drugs (Pagana 2021). Cognitive dysfunction is also a primary symptom of hypothyroidism (Ettleson 2020). Metabolically, thyroid insufficiency is associated with decreased energy expenditure, weight gain, and increased cholesterol levels (Mullur 2014).
High total T4 is associated with hyperthyroidism, acute thyroiditis, thyroid cancer, toxic goiter, congenital hyperproteinemia, elevated thyroid binding globulin, hepatitis, pregnancy, and certain medications, including estrogen, oral contraceptives, clofibrate, heroin, and methadone (Pagana 2021). Metabolically, excess thyroid hormone increases energy expenditure, weight loss, and decreased cholesterol (Mullur 2014). Exposure to organophosphate pesticides (Fortenberry 2012) and phthalates (Choi 2020 ) may increase total T4.
Thyroxine (T4), also known as thyroxine or tetraiodothyronine, is the primary hormone secreted by the thyroid gland. It contains four atoms of iodine and represents 90% of the total thyroid hormone released. Approximately 99% of T4 in circulation is considered inactive as it is bound to proteins, including thyroid-binding globulin, albumin, and transthyretin (prealbumin). Total T4 measurement reflects bound and free T4, with free T4 best-reflecting thyroid status. Hormone replacement therapy and pregnancy can increase thyroid binding globulin (TBG) and falsely elevate total T4, suggesting that hyperthyroidism is present when it is not. A reduction in TBG, e.g., hypoproteinemia, may falsely decrease total T4 even though hypothyroidism is absent (Pagana 2021).
Approximately one-third of circulating T4 is converted peripherally to active T3, while one-third is normally converted to reverse T3. Reverse T3 is inactive though some can be converted to active T3 (Peeters 2017). Total T4 will be elevated in overt hyperthyroidism but can be within the conventional range in subclinical hyperthyroidism despite a low TSH. Total T4 will be decreased in overt hypothyroidism but can be within the traditional range with an elevated TSH in subclinical hypothyroidism (IOM 2003).
Thyroid hormone production and levels are regulated by TSH from the pituitary and TRH from the hypothalamus, as well as by nutritional signals such as leptin and appetite-regulating peptides. Thyroid hormone directly affects the metabolism of carbohydrates, protein, and lipids; influences energy expenditure and storage; maintains basal metabolic rate; and controls appetite and food intake. Thyroid hormone also modulates insulin sensitivity and gluconeogenesis in the liver and directly affects the brain, fat tissue, skeletal muscle, and pancreas. Excess thyroid hormones contribute to increased resting metabolic rate, weight loss, decreased cholesterol, increased lipolysis, and increased gluconeogenesis. Insufficiency of thyroid hormone is associated with decreased resting energy expenditure, increased weight, increased cholesterol, decreased lipolysis, and reduced gluconeogenesis. (Mullur 2014).
Thyroid hormones influence several cardiac functions, including heart rate, output, and vascular resistance, and thyroid abnormalities are often observed in congestive heart failure, cardiac ischemia, and associated oxidative stress (Moura 2016).
Administration of exogenous T4 can resolve hypothyroidism and its related cognitive symptoms, including impaired memory and concentration, altered perceptual and executive function, and changes in language and psychomotor function. However, for those whose symptoms do not resolve, a combination of T4 and T3 may be indicated (Ettleson 2020).
While thyroid hormone production relies on sufficient iodine, the deiodinase conversion of T4 to T3 relies on sufficient selenium. An increase in free T4 and a decrease in free T3 may be observed with selenium insufficiency (Kobayashi 2021).
It is essential to evaluate micronutrient status in general when assessing thyroid function, including zinc, copper, iron, molybdenum, and vitamin A. Insufficiency of any of these nutrients, as well as iodine and selenium, may impair thyroid metabolism (O’Kane 2018). The conditionally essential amino acid tyrosine is also required for thyroid hormone synthesis.
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Kobayashi, Ryohei, et al. "Thyroid function in patients with selenium deficiency exhibits high free T4 to T3 ratio." Clinical Pediatric Endocrinology 30.1 (2021): 19-26.
Moura Neto, Arnaldo, and Denise Engelbrecht Zantut-Wittmann. “Abnormalities of Thyroid Hormone Metabolism during Systemic Illness: The Low T3 Syndrome in Different Clinical Settings.” International journal of endocrinology vol. 2016 (2016): 2157583. doi:10.1155/2016/2157583
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