Hypothalamic-Pituitary-Thyroid Axis - Podcast Version TeachMePhysiology 0:00 / 0:00 1x 0.25x 0.5x 0.75x 1x 1.25x 1.5x 1.75x 2x The hypothalamic-pituitary-thyroid (HPT) axis consists of the hypothalamus, the pituitary gland, and the thyroid gland. It regulates the production and release of thyroid hormones which, in turn, regulate metabolic homeostasis throughout the body. This includes expenditure, cardiovascular function and neurological activity. This article will discuss the function of the HPT axis, how it is regulated, and its clinical relevance. Further information on the thyroid gland and thyroid hormone synthesis can be found here. Pro Feature - 3D Model You've Discovered a Pro Feature Access our 3D Model Library Explore, cut, dissect, annotate and manipulate our 3D models to visualise anatomy in a dynamic, interactive way. Learn More Function of the HPT Axis Four hormones are involved in the HPT axis: Thyrotropin-releasing hormone (TRH) secreted by the hypothalamus Thyroid-stimulating hormone (TSH) released by the anterior pituitary Thyroid hormones T3 (triiodothyronine) and T4 (tetraiodothyronine or thyroxine) produced by the thyroid gland. The widespread physiological effects of the HPT axis are exerted through thyroid hormones T3 and T4. Effects of T3 and T4 Key functions of the thyroid hormones are included in the table below: System/Organ Overall Effect Mechanistic detail Metabolic Increased basal metabolic rate and catabolic activity Promotes lipolysis, glycogenolysis, glycolysis, proteolysis Nervous system Improved cognition and increased reflex speed Supports neural development, metabolism and cognitive function Cardiovascular Increased cardiac output Increased cardiac muscle protein synthesis Bone Increased bone turnover (shortened remodelling cycle leaves less time for mineralisation) Promotes linear growth and development, increased osteoblast and osteoclast activity Regulation of the HPT Axis Hypothalamic Control The axis begins with thyrotrophin-releasing hormone (TRH) production by the hypothalamus. TRH binds to receptors on thyrotroph cells in the anterior pituitary gland. This stimulates the synthesis and release of thyroid-stimulating hormone (TSH), also known as thyrotropin. TSH then enters the circulation and binds to receptors on follicular cells within the thyroid gland. This stimulates the synthesis and release of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Regulation of Thyroid Hormones Thyroid hormones are regulated by negative feedback within the HPT axis. High circulating levels of thyroid hormones suppress further hormone release. Specifically, increased T3 and T4 inhibit both TRH secretion from the hypothalamus and TSH secretion from the anterior pituitary. At the level of the anterior pituitary, T3 enters thyrotroph cells either as circulating T3 or the conversion of circulating T4 by deiodination. Two important mechanisms then contribute to its regulation: Reduced TRH receptors – intracellular T3 reduces the number of TRH receptors making them less responsive to signals from the hypothalamus Reduced TSH secretion – T3 binds to response elements on TSH promotor regions within the DNA, directly inhibiting transcription of TSH, leading to less stimulation of the thyroid gland. Overall, this results in reduced stimulation of the thyroid gland and therefore decreased thyroid hormone production. Created in https://BioRender.com Figure 1Feedback mechanisms involved with thyroid hormones. Clinical Relevance Abnormal Thyroid Function Disorders of the thyroid gland commonly result from disruption of the HPT axis, leading to either excessive or insufficient thyroid hormone production. Two major clinical states occur: hyperthyroidism, where excessive thyroid hormones are produced, and hypothyroidism, where thyroid hormone production is inadequate. Symptoms Patients with thyroid disease often present with symptoms affecting multiple organ systems, related to the normal function of thyroid hormones. The table below summarises common features of hyperthyroidism and hypothyroidism. Feature Hyperthyroidism Hypothyroidism Metabolic Weight loss despite normal appetite Weight gain Cardiovascular Tachycardia, palpitations, atrial fibrillation Bradycardia Nervous system Anxiety, irritability, tremor Fatigue, slowed cognition Temperature tolerance Heat intolerance Cold intolerance Skin and hair Warm, moist skin. Fine hair Dry skin. Hair thinning Gastrointestinal Increased bowel frequency Constipation Reproductive Menstrual irregularities Menorrhagia (heavy menstrual bleeding) Neuromuscular Hyperreflexia Slowed reflexes Hyperthyroidism is most commonly caused by Graves’ disease, an autoimmune condition in which antibodies stimulate the TSH receptor, leading to increased thyroid production. Other causes include toxic multinodular goitre and thyroiditis. Hypothyroidism is most commonly caused by autoimmine thyroiditis (Hashimoto’s thyroiditis), where immune-mediated destruction of thyroid tissue reduces hormone production. Thyroid dysfunction may also produce cardiovascular complications such as heart failure due to the increased metabolic demand placed on the heart. Diagnosis and treatment The function of the thyroid gland is measured using a blood test called thyroid function tests (TFTs), which measure circulating levels of TSH, T3 and T4. Treatment for hyperthyroidism typically involves medications that reduce thyroid hormone synthesis, such as carbimazole. Beta-blockers (e.g. propranolol) may also be used to control symptoms such as palpitations and tremor. Some cases may also require surgical intervention to remove part or all of the thyroid gland. Treatment of hypothyroidism involves replacement of deficient thyroid hormones with levothyroxine. 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