Endocrinology and Metabolism Clinics of North America
Hypothyroidism
Section snippets
Epidemiology
Hypothyroidism is a relatively common disorder. The prevalence of hypothyroidism increases with age, and the disorder is nearly 10 times more common in females than in males. Hypothyroidism is particularly common in areas of iodine deficiency. Individuals who have thyroid peroxidase antibodies and those who have thyroid-stimulating hormone (TSH) values that are in the upper normal range are at increased risk for developing hypothyroidism.
The prevalence of overt hypothyroidism varies according
Etiology
A summary of the most common causes of hypothyroidism is given in Box 1.
Resistance to thyroid hormones
Hypothyroidism may be transient or permanent, central, or primary. Central hypothyroidism can accompany disorders of the hypothalamic-pituitary axis, leading to reduced TSH secretion or reduced biological activity of TSH. As a result, there is reduction in thyroid stimulation by the TSH and, secondarily, reduced thyroid hormone synthesis and secretion.
Primary hypothyroidism refers to a defect in the thyroid gland resulting in reduced synthesis and secretion of thyroid hormones.
Clinical presentation
The scope of thyroid hormone deficiency encompasses the different body systems and organs. The clinical presentation of a patient who has hypothyroidism depends on the severity of the condition. This depends on the degree of biochemical hypothyroidism. There is significant individual variation. Some patients present with mild symptoms in spite of having low levels of circulating thyroid hormones. Others who have less pronounced biochemical hypothyroidism may be more symptomatic. This is also
Diagnosis
The diagnosis of hypothyroidism is based on the combination of clinical context and laboratory tests. Imaging of the brain and pituitary gland is required for patients in whom central hypothyroidism is suspected.
In the majority of patients, making the diagnosis of hypothyroidism should not be complicated. A number of factors can affect the levels of TSH, total T4, and total T3; in particular, several medical conditions can increase or decrease the concentration of total T4 and total T3 through
Treatment of hypothyroidism
Hypothyroidism can cause considerable morbidity. The treatment of hypothyroidism is, in principle, simple. Synthetic thyroxine is the preferred form of thyroid hormone replacement therapy. Hypothyroidism in the majority of patients is permanent and should be treated lifelong. The main exceptions are patients who have transient hypothyroidism due to subacute thyroiditis and patients who have drug-induced hypothyroidism. These patients should be treated during their hypothyroid phase.
The main
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Sensitivity of Anguilliformes leptocephali to metamorphosis stimulated by thyroid hormone depends on larval size and metamorphic stage
2023, Comparative Biochemistry and Physiology -Part A : Molecular and Integrative PhysiologyCitation Excerpt :This result implies that larger larvae are more responsive to T4; conversely, smaller leptocephali may not be able to induce normal metamorphosis or are at increased risk of death due to slow or weak responsiveness to TH. In mammals, hypothyroidism impairs linear growth, delays endochondral ossification, and reduces mineralization, whereas hyperthyroidism leads to osteoporosis, early fusion of bones, and increased mineral deposition (Devdhar et al., 2007; Bassett and Williams, 2016; De Leo et al., 2016). Furthermore, aberrant levels of TH are associated with changes in bone density and increased risk of bone fracture (Vestergaard and Mosekilde, 2002; Vestergaard et al., 2005).
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