Endocrine System - Lecture Twenty Seven : Thyroid, Parathyroid and Adrenal Glands
9 important questions on Endocrine System - Lecture Twenty Seven : Thyroid, Parathyroid and Adrenal Glands
What are thyroid hormones ?
- They are different to other lipid-soluble hormones (steroid hormones) in the fact that they are made in advance and stored until required
- The hormone-receptor complex binds to DNA and acts as a transcription factor – leading to a specific protein being synthesised.
- The effects are slow to develop (hours-days)
- There are two thyroid hormones:
- T4 is the inactive form, but is produced in higher quantities
- T4 is converted to T3 when required
Explain How target cells are activated by thyroid hormones ?
- Thyroid follicles contain a protein called thyroglobulin (TGB). Iodine (from our diet) reacts with TGB molecules in the follicles.
- Iodised TGB moves to the follicular cells where thyroid hormones (T3 and T4) detach from TGB.
- Thyroid hormones are stored* in follicular cells until required
- When stimulated by thyroid stimulating hormone (TSH), T3 and T4 are secreted into the blood to target cells
- Iodine deficiency: as iodine is necessary for thyroid hormone production, a deficiency of iodine in our diet results in low levels of thyroid hormone in the body
What are the main effects of thyroid hormones?
- Growth
- Alertness
- Metabolism
- Increases body heat production (increases oxygen consumption and ATP hydrolysis)
- Stimulates fatty acid oxidation (usage of stored fat) in many tissues.
- Increases proteolysis, predominantly from muscle.
- Stimulates carbohydrate metabolism (usage of stored glucose), enhances insulin-dependent entry of glucose into cells, increases gluconeogenesis and glycogenolysis
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What are examples of thyroid hormone disorders ?
- Caused by hyposecretion of thyroid hormone
- Symptoms include low metabolic rate, weight gain, cold intolerance, lethargy, slow heart rate, growth retardation (cretinism only), hindered brain development (cretinism only)
- Caused by hypersecretion of thyroid hormone
- Symptoms include high metabolic rate, weight loss, heat intolerance, nervousness, increased heart rate, tissue swelling
Thyroid hormone negative feedback loop
- Stimulus: CNS input
- Hypothalamus: secrets thyroid releasing hormone (TRH)
- Anterior pituitary: secretes thyroid stimulating hormone (TSH)
- Thyroid gland: secrets thyroid hormones (T3 and T4)
- Effects:
- Increases body heat production (increased oxygen consumption and ATP hydrolysis)
- Adipose: increases fatty acid mobilisation
- Muscle: increases glycogenolysis and proteolysis
- Liver: increases glycogenolysis and gluconeogenesis
- Other effects: stimulates growth (foetus and early childhood), maintains normal alertness and reflexes in the nervous system
- Negative feedback : T3 and T4 negatively feedback on the anterior pituitary (decrease TSH), and the hypothalamus (decrease TRH)
What is the function and location of the parathyroid glands?
- Location : Posterior aspect of the thyroid gland
- Function: Secretes parathyroid hormone which is essential for life. It acts to increase blood calcium
Parathyroid hormone negative feedback loop
- Stimulus : low blood calcium concentration
- Parathyroid gland : secrets PTH
- Effects:
- Activate vitamin D --> increased calcium absorption from food
- Bone: increase bone breakdown (OC > OB) which increase the release of calcium into blood
- Overall response : increases blood calcium back towards the set point
- Negative feedback : normal blood calcium negatively feeds back to the parathyroid gland to reduce secretion of PTH
What is the structure, function and location of adrenal glands?
- Location : superior to each kidney
- Structure : Made up of two separate glands. Has an adrenal cortex (outer region has 3 layers) which secretes steroid hormones
- Functions :
- Middle layer : secretes cortisol
- Inner layer : secretes androgens
What is the structure and location of a thyroid gland? What hormones are made and released?
- Location : Below the larynx, on the anterior and lateral surfaces of the trachea
- Structure :
- Hormones made and released : Thyroid hormones which is essential for metabolic activity. Calcitonin hormone which is involved in calcium homeostasis
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