Lecture 20: Thyroid Hormones Flashcards by Matthew Wandabwa

what joins the two lobes of the thyroid gland

section of tissue called isthmus

How well did you know this?

Not at all

Perfectly

what is the thyroid gland comprised of

lobules of spherical follicles filled with colloid

How well did you know this?

Not at all

Perfectly

what type of cells make up the follicles present in thyroid lobules

follicular cells –> cuboidal epithelial cells

How well did you know this?

Not at all

Perfectly

what is the primary constituent of colloid

thyroglobulin

How well did you know this?

Not at all

Perfectly

what is incorporated into a thyroglobulin molecule

residues of amino acid tyrosine

How well did you know this?

Not at all

Perfectly

what is the function of the C (parafollicular) cell

produces hormone calcitonin

How well did you know this?

Not at all

Perfectly

where does thyroid hormone synthesis take place

in colloid

How well did you know this?

Not at all

Perfectly

what molecule acts as a scaffold for thyroid hormone synthesis

thyroglobulin

How well did you know this?

Not at all

Perfectly

what are the basic ingredients for thyroid hormone synthesis

tyrosine and iodine

How well did you know this?

Not at all

Perfectly

which ingredient of thyroid hormone synthesis needs to be consumed in the diet

iodine

How well did you know this?

Not at all

Perfectly

give examples of food that contains iodine

sea vegetables
iodised salt
eggs
cranberries
seafood

How well did you know this?

Not at all

Perfectly

consequence of low iodine in diet

hypothyroidism

How well did you know this?

Not at all

Perfectly

outline how thyroxine (T4) and triiodothyronine (T3) are formed

I + tyrosine = MIT
MIT + I = DIT
DIT + DIT = T4
DIT + MIT = T3

How well did you know this?

Not at all

Perfectly

explain how iodine gets into the follicle cell and then colloid in thyroid

iodine reduced by digestion in stomach to iodide
co-transported with Na+
secondary active transport using energy from Na+/K+ ATPase pump
2 Na+ ions transported with 1 I-
move through follicle cell by diffusion
I- transported across apical membrane via I-/Cl- transporter into lumen (colloid)

How well did you know this?

Not at all

Perfectly

how is T3/ T4 formed in the colloid

iodide is oxidised back to iodine by hydrogen peroxide
thyroid peroxidase (TPO) attaches iodines to tyrosine in thyroglobulin
coupling of DIT/MIT to give T3 or T4

How well did you know this?

Not at all

Perfectly

what is the use of radioactive iodine

diagnostic, tracer, and therapeutic (causes cells to die in hyperthyroidism

How well did you know this?

Not at all

Perfectly

which thyroid hormone is produced most in thyroid gland

T4 (80-90%)

How well did you know this?

Not at all

Perfectly

which thyroid hormone is more biologically active

T3 - binds with greater affinity to nuclear receptors

how is T4 converted to T3

T4 needs to be deionised by deiodinase

what are the types of deiodinase and where do they come from

type 1 –> liver, kidney, thyroid, pituitary, heart

type 2 –> pituitary, brain, skeletal muscle, brown adipose

type 3 –> placenta, uterus, brain, kidney, liver, thyroid, fatal tissues

where does most systemic T3 come from

liver and kidney by type 1 deiodinase

which molecule is responsible inactivation of T3

type 3 deiodinase

how are thyroid hormones transported in plasma

- thyroid hormones are lipophilic 
transported by:
--> thyroid binding globulin (TBG) 
--> thyroid binding pre albumin (TBPA) 
--> thyroid binding albumin (TBA)

tiny % of thyroid hormone unbound in blood

which thyroid hormone being transported is biologically active

free unbound thyroid hormone

where are majority of thyroid receptors located

in nuclei of cells

how does thyroid hormone cause suppression or activation of genes in a cell

- binds to receptors in nucleus of a cell causing genomic effect - translocation to hormone response element on DNA - proteins produced after suppression/activation of genes

function of proteins after suppression/activation of genes by thyroid hormone

increase metabolism within cell therefore thyroid hormones increase basal metabolic rate

how do thyroid hormones increase ATP production

bind to receptors directly on mitochondria

describe the metabolic actions of thyroid hormones

- increased O2 consumption (required by proteins produced) - increased BMR (basal metabolic rate) - calorigenic effect --> thermogenic effect - carbohydrate metabolism - -> ^ absorption of glucose from GIT, ^ glycogenolysis, ^ gluconeogenesis - lipid metabolism - -> lipolysis...^ circulatiing FFA, ^ FFA oxidation, decrease cholesterol, ^ No. of LDL receptors - protein metabolism - -> prot. synthesis and breakdown

describe systemic effects of thyroid hormones

Heart - -> ^ HR, CO, SBP and decrease DBP (sympathomimetic effect) - -> ^ blood flow to skin Lungs --> ^ vent. rate GIT - -> ^ appetite - -> ^ secretion digestive juices - -> ^ GIT motility Reproductive --> essential for normal reproduction and lactation MSK - -> promotes normal body growth and maturation of skeleton - -> promotes normal body function and development of muscles NS - -> promotes normal neuronal development in foetus and infant - -> promotes normal neuronal function in adult (^ synaptic activity) - -> ^ effects of SNS - sympathomimetic; up regulates B1 adrenergic receptors in heart

what effect does cold have on production of TRH in hypothalamus

+ve

what effect does stress have on production of TRH in hypothalamus

-ve --> prevents overstimulation of SNS

how does TSH act on thyroid gland

- ^ iodide trapping and binding (by ^ gene transcription of Na+/I- symporter) - promotes thyroglobulin synthesis and secretion into colloid - TPO synthesis - stimulates T3 and T4 synthesis - promotes colloid endocytosis into follicular cells - ^ size and No. of follicular cells - ^ blood flow at thyroid level

what do primary/secondary/tertiary levels of hypothyroidism refer to

1st - problem at thyroid (failure to respond to TSH) 2nd - problem at pituitary (w/ TSH); deficient TSH secretion --> thyroid atrophy (shrinking) 3rd - problem at hypothalamus (w/ TRH) --> deficient TSH secretion due to deficient TRH secretion

what is hypothyroidism

deficient production of thyroid hormone

name some causes of primary hypothyroidism

- thyroiditis e.g. Hashimoto's disease or chronic lymphocytic thyroiditis (CLT) - severe iodine deficiency - severe deficiency of one or more synthesis enzymes - removal or destruction of thyroid gland

name some consequences of primary hypothyroidism

- elevated TSH levels --> reduced -ve feedback | - thyroid enlarges --> goitre formation

how is congenital hypothyroidism diagnosed

blood test performed on all newborns (ESSENTIAL EARLY DIAGNOSIS)

describe developmental consequences of hypothyroidism in baby

- sometimes enlarged tongue - unhappy - difficulty feeding - difficulty w/ GIT

describe developmental consequences of hypothyroidism in adult

- short - impaired brain, bone and other development - poor coordination

what happens if you don't have a functional thyroid gland

thyroid hormone is replaced

name causes of hyperthyroidism / thyrotoxicosis

- autoimmune disease --> Grave's disease - thyroid adenoma - TSH secreting adenoma

how does Grave's disease cause hyperthyroidism

- autoantibodies stimulate thyroid gland - bind to same receptors as TSH - thyroid gland secrete excess T3 and T4

diagnostic factors of Grave's disease

- very low TSH - goitre - exophthalmos - retraction of upper eyelid