what does the complex process of growth of an ovum to adult involve?
hyperplasia and hypertrophy (increase in size and number of cells)
what does the complex process of growth of an ovum to an adult depend upon?
- gene contribution
- nutrient
- hormones regulating linear growth
- regulation of body mass
what regulates body mass?
- leptin: regulates CNS feeding behaviour
- ghrelin: promotes appetite
what are the different types of thyroid hormone and what do they do?
- thyroxine (T4)
- triiodothyronine (T3)
- they regulate transcription of cell proteins by rating on nuclear receptors
- they are essential for the growth, development and metabolism of tissues
describe the structure of the thyroid gland
- thyroid gland is made up of many follicles.
- each follicle is filled with colloid.
- the walls of each follicle are made up of follicular cells which trap iodine, essential for thyroid production.
what % of the secreted thyroid are T4 and T3
- 90% T4
- 10% T3
- however, T3 is much more active
what metabolises T4 and T3?
- non-thyroidal tissues (kidney and liver)
how do thyroids travel in blood?
- 99% bound to plasma proteins
- such as thyroid-binding globulin, albumin and TTR
- they act as a buffer for bound thyroids
- they prolong the half-lives of thyroids
describe the action of thyroid hormones on their target cells
- the unbound t3 and t4 diffuse across the PM
- the t4 converts into t3 in the cytosol
- they attach to the thyroid hormone receptor (TR), on the thyroid response element (TRE) of the DNA
- they affect the transcription of cell proteins
what does thyroid hormone increase and how
thyroid hormone increases the basal metabolic rate (BMR) by stimulating catabolic and anabolic reactions in pathways affecting carbohydrates, proteins and lipids.
why is there a 1 0 day delay after thyroid injection, for the BMR to peak?
takes time for gene expression levels to change.
what are the actions of thyroid hormone?
- increase protein metabolism
- increase lipid metabolism
- promotes futile cycles (reactions which release heat0
- these changes require changes in the expression of several genes.
explain cretinism
- thyroid is essential for normal growth and development.
- deficiencies in iodine or defects in thyroid synthesis causes cretinism
- leads to mental retardation, short height, coarse hair and protuberant abdomen
describe the pathway the body takes for thyroid hormone to be released
- hypothalamus releases thyrotropin-releasing hormone (TRH)
- stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH)
- stimulates the thyroid gland to secrete T4 and T3.
- results in the increase of metabolic rate and heat production
what is graves disease?
- hyperthyroidism
- cause: abnormal immunoglobin, TSI - thyroid-stimulating immunoglobin, leads to increased production of T4 and T3, and thyroid enlarges to form a goitre.
- symptoms include: increased metabolic rate, weight loss, rapid heartbeat, excessive heat production (sweating), some can develop a double vision and exophthalmos (forward protrusion of eyes)
what is hypothyroidism?
- most common endocrine disease
- most common cause is iodine deficiency
- if left untreated: goitres form, skin changes, constipation, fatigue and headaches
