T1 L7 Thyroid hormones and physiology Flashcards
Describe the tissue of the thyroid gland
Composed of follicles consisting of monolayer of epithelial cells enclosing a large core of viscous homogenous colloid
What does the colloid do?
Thyroid hormones are stored here
What are the 3 primary hormones secreted by the thyroid gland?
T3
T4
Calcitonin
What does T3 stand for?
Triiodothyronine
What does T4 stand for?
Tetraiodothyronine
What is calcitonin involved in?
Calcium homeostasis
Secreted independently of other thyroid hormones
Where does iodine come from?
The diet
Vegetables and meat providing the animals have been fed on vegetables containing iodine
What can affect thyroid function?
Pollution
Contamination
Iodine deficiency
Iron deficiency
What degrades MIT & DIT to free the iodide?
Halogenases
What happens when T3 and T4 leave the follicular cells?
Enter blood stream and are distributed to target tissues
What percentage of thyroid hormone leaving the thyroid gland is T4?
95%
Why can’t we be certain that T3 and T4 inhibit TRH release in humans?
We can’t measure TRH in humans
What happens if you are iodine deficient?
1) Thyroid hormone decreases
2) Lose negative feedback
3) Increased TSH
4) Enlarged thyroid gland
How are thyroid hormones carried round the body?
Bound to plasma proteins and transported in the blood
Describe the binding of T4
75% to thyronine-binding globulin (TBG)
15-20% to thyroxine
5-10% to albumin
What happens after a thyroid hormone has interacted with its intracellular receptor?
Influence gene transcription to influence protein synthesis
What is the main effect of thyroid hormones?
Increase basal metabolic rate
How do thyroid hormones increase the basal metabolic rate?
Increase in carbohydrate metabolism
Increase in synthesis, mobilisation and degradation of lipids
Increase in protein synthesis
How is carbohydrate metabolism increased by thyroid hormones?
Glycogenesis and glucose uptake by muscle and adipose cells
Potentiation of effects of insulin
Potentiation of effects of catecholamines
Increase in glucose absorption by GI tract
What organs does thyroid hormone not increase the basal metabolic rate of?
Brain Uterus Testes Spleen Thyroid gland Anterior pituitary gland
What happens to the growth of an individual who is thyroid hormone deficient?
They won’t grow and will continue looking like a child after puberty
What happens to the growth of an individual who is growth hormone deficient?
They won’t grow but they will undergo normal changes of ageing so they look like a short adult
What is levothyroxine used for?
Treat thyroid deficiency
Suppress TSH secretion in treatment of some thyroid tumours
How is levothyroxine administered?
Mouth
Injection
What are the adverse effects of levothyroxine?
palpitations arrhythmias diarrhoea insomnia tremor weight loss
What drugs affect thyroid function?
Carbimazole and methimazole Propythiouracil Potassium perchlorate Radioactive iodine Lithium Iodides
What drugs can induce goitre?
Lithium
Iodides
What does potassium perchlorate do?
Competes with iodide for active iodide uptake pump
Describe prophylactic administration of iodine
Injection or incorporation into table salt or flour
Reduced incidence of endemic goitre
Carries risk of Jod-Basedown phenomena
What is carbimazole used for?
Hyperthyroidism
What are the adverse effects of carbimazole?
Rashes and pruritus
Neutropenia
Agranulocytosis
Teratogenic
How is carbimazole a pro drug?
After absorption it is converted into active form methimazole which prevents peroxidase iodinating the tyrosine residues on thyroglobulin which reduces production of thyroid hormones T3 and T4
What is propylthlouracil used for?
Hyperthyroidism including Graves disease
Drug of choice in first trimester
What are the adverse effects of propylthouracil?
Rashes and pruritus
agranulocytosis
Serious liver injury
