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Synthesis and Regulation of Thyroid Hormones

Synthesis and Regulation of Thyroid Hormones

Low blood levels of the thyroid hormones — triiodothyronine (T3) and thyroxine (T4) — signal the hypothalamus to release the thyrotropin-releasing hormone (TRH). TRH then reaches the pituitary gland and stimulates the release of thyroid-stimulating hormone(TSH) into the bloodstream.
Upon reaching the thyroid gland, TSH stimulates the follicular cells' active uptake of iodide ions from the blood. The ions diffuse to the apical surface of the cells and are oxidized to iodine. The iodine is then...

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The thyroid hormone (TH) plays a pivotal role in the intricate orchestration of physiological processes, exerting profound effects on development, metabolism, and homeostasis throughout different life stages.
TH is indispensable for the normal development and maturation of the skeletal, muscular, and nervous systems during fetal and childhood growth. It facilitates bone mineral turnover and regulates protein synthesis in developing tissues, contributing significantly to overall growth and...

Hyperthyroidism I: Introduction

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Hyperthyroidism is a type of thyrotoxicosis characterized by the thyroid gland's overproduction of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). This hormone excess increases the basal metabolic rate and enhances sensitivity to catecholamines.DiagnosisDiagnosis is based on clinical features and biochemical testing. It typically shows suppressed thyroid-stimulating hormone (TSH) levels below 0.4 mIU/L, with elevated free T3 and/or T4. Additional tests, including thyroid...

Hyperthyroidism II: Pathophysiology

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Hyperthyroidism is a hypermetabolic state caused by elevated levels of thyroid hormones, triiodothyronine (T3) and thyroxine (T4). It results from dysregulation at the thyroid, pituitary, or immune system level and affects multiple organ systems.PathophysiologyThe most common cause of hyperthyroidism is Graves’ disease, an autoimmune disorder in which antibodies, specifically thyroid-stimulating antibodies (TSAb), a subtype of TSH receptor antibodies (TRAb), bind to and activate TSH receptors...

Graves Disease II: Pathophysiology

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Graves’ disease is an autoimmune disorder characterized by the production of thyroid-stimulating immunoglobulins (TSI) that activate TSH receptors, leading to excessive synthesis and release of thyroid hormones (T3 and T4) and resulting in hyperthyroidism.Among all causes of hyperthyroidism, Graves’ disease is the most common and can happen at any age, though it is more frequent in women. It produces a hypermetabolic state with features such as weight loss, tachycardia, tremor, and heat...

Hypothyroidism II: Pathophysiology

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Hypothyroidism is a disorder characterized by insufficient production of thyroid hormones, which regulate metabolism, energy balance, and multiple organ systems.TypesHypothyroidism is classified based on the level of dysfunction. Primary hypothyroidism results from intrinsic thyroid gland dysfunction, causing reduced hormone production despite normal or increased stimulation. Secondary hypothyroidism arises from inadequate thyroid-stimulating hormone (TSH) secretion by the pituitary. Tertiary...

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Related Experiment Video

Updated: Jul 12, 2026

Substernal Thyroid Biopsy Using Endobronchial Ultrasound-guided Transbronchial Needle Aspiration

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Thyroid hormone replacement for subclinical hypothyroidism.

H C C E Villar¹, H Saconato, O Valente

¹Faculdade Estadual de Medicina de Marília, Departamento de Medicina Interna, Av. Cascata 123, Marília, São Paulo, Brazil, 17515-300. hvillar@famema.br

The Cochrane Database of Systematic Reviews

|July 20, 2007

Summary

Thyroid hormone replacement for subclinical hypothyroidism did not improve survival or cardiovascular health. While some lipid profiles and heart function showed minor improvements, quality of life and symptoms did not significantly change.

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Substernal Thyroid Biopsy Using Endobronchial Ultrasound-guided Transbronchial Needle Aspiration

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Published on: November 10, 2014

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Area of Science:

Endocrinology
Internal Medicine

Background:

Subclinical hypothyroidism, characterized by elevated TSH with normal thyroid hormones, affects 4-8% of the general population.
Prevalence increases to 15-18% in women over 60, with ongoing debate about treatment necessity and clinical significance.

Purpose of the Study:

To evaluate the effects of thyroid hormone replacement therapy in individuals with subclinical hypothyroidism.

Main Methods:

Systematic review of 12 randomized controlled trials (RCTs) involving 350 adults with subclinical hypothyroidism.
Trials compared levothyroxine replacement with placebo or no treatment, with a minimum follow-up of one month.

Main Results:

No significant improvements in survival or cardiovascular morbidity were observed.
No statistically significant differences in quality of life or symptoms were found, though one study noted improved cognitive function.
Trends suggested improvements in lipid profiles and echocardiographic parameters, including myocardial relaxation and diastolic function.

Conclusions:

Current RCTs indicate levothyroxine replacement does not enhance survival or reduce cardiovascular morbidity in subclinical hypothyroidism.
No significant impact on health-related quality of life or symptoms was demonstrated.
Evidence suggests potential benefits for lipid profiles and left ventricular function.