thyroid gland Flashcards
what types of hormones are there?
peptides
steroids
amines
what type of hormone is thyroid hormone?
amine
what are the 2 main types of thyroid hormone?
Thyroxine (T4) and Triiodothyronine (T3)
which thyroid hormone is the active form?
T3
which thyroid hormone is made in the largest amounts by the thyroid?
T4
which lobe of the thyroid is larger?
the right lobe
how will enlargement of the thyroid present?
right sided lump in the neck
why is the thyroid so good at trapping iodine in a short amount of time?
Receives the most blood per unit volume – hence why it’s so good at trapping iodine in a short period of time
what are thyroid follicles?
Thyroid follicles are a ring of cells surrounding a ball of fluid in the lumen (colloid)
what type of cells are thyroid cells?
cuboidal
columnar when active
name some adaptations of the thyroid follicular cell?
• Apex has numerous microvilli extending into the colloid
• Cytoplasm has extensive ER and microsomes
• Predominant Golgi apparatus bc thyroglobulin has a carbohydrate portion
• Numerous lysosomes to break down Tg
- TSHR to receive pituitary signals
what is NIS?
thyroidal sodium iodine symporter) to take up iodine from the circulation
what is pendrin?
protein to transport iodine into the colloid
what is thyroid peroxidase’s function?
oxidising the iodine
what is iodotyrosine dehalogenase 1 (IYD) needed for?
recycling iodine
how is thyroid globulin secreted into the follicle lumen?
via exocytosis
what is thyroglobulin?
a long peptide chain with lots of tyrosine rings
what happens if pendrin is mutated?
hypothyroidism and sensory neuron deafness bc it’s also expressed in the semi-circular canals
why does thyroperoxidase do?
converts I- to atomic iodine
what is MIT?
monoiodotyrosine
when atomic iodine attaches to just 1 tyrosine ring on the thyroglobulin
what is DIT?
diiodotyrosine
when atomic iodine attaches to 2 tyrosine rings on the thyroglobulin
how is T3 made?
DIT + MIT
how is T4 made?
DIT + DIT
how is T4 converted to T3?
di-iodinase enzymes
what signalling pathways are activated when TSH binds to TSHR?
cAMP or IP3
where is D2 expressed?
brain, liver and brown fat
what happens to D2 in hypothyroidism?
D2 enzyme is upregulated so more T4 T3 – acts as a buffer against changes in T4 levels
when is D2 upregulated in brown fat?
when you’re cold bc T3 stimulates thermogenesis
what proteins do thyroid hormones bind to when in circulation?
thyroid binding globulin or albumin
what happens to T3 and T4 levels in pregnancy? explain how this happens
• In pregnancy, oestrogen levels are higher – causes attachment of side chains to TBG (scialation). Increases the half-life of TBG binds more thyroid hormone less free thyroid hormone hypothyroidism decreased T4 levels release more thyroid hormone to compensate
what are the locations for receptors?
nucleus
cytoplasm
cell surface
what happens to the thyroid hormone receptor when thyroid binds?
- When thyroid hormone binds to THR, THR complex falls apart
* Retinoic acid is recruited to make a new heterodimer and allows transcription to be activated
what is hyperthyroidism?
when the thyroid gland makes too much thyroid hormone
what is thyrotoxicosis?
the state in which there’s too much thyroid hormone
what can cause thyrotoxicosis?
Can be caused by hyperthyroidism; damage to the thyroid gland and releasing stored thyroid; taking too much thyroid
what are the effects of hyperthyroidism of the cardiovascular system?
- Basal metabolic rate increases increases heat production
- More blood is sent to the skin so you get warmer
- Need to increase CO and therefore increase HR
- Can lead to tachyarrhythmias
how does hyperthyroidism affect metabolism?
- Heat intolerance
- Increased basal metabolic rate
- Increased appetite bc nutrients needed to supply fuel
- Need to break down protein and lipids to get this fuel
- Leads to weight loss and myopathy (mostly felt in larger proximal muscles – cant stand up in a chair)
what is the effect of hyperthyroidism on diabetes?
o Increased insulin turnover
o Increased gluconeogenesis
o Reduced insulin secretion
what effect does hyperthyroidism have on the nervous system?
- Nervousness
- Seizures – lower seizure threshold in people with epilepsy
- Hyperphagia – increased hunger
what effect does hyperthyroidism have on the skin?
Plummer’s nails
Pretibial myxoedema
what is pretibial myxoedema?
a swollen rash you get in Graves disease
what effect does hyperthyroidism have on the GI tract?
• Increased appetite, weight loss, increased motility, transaminitis, constipation
what effect does hyperthyroidism have on bones?
- Accelerated osteoclast activity – break down bone for fuel transient hypercalcaemia
- Osteoperosis in long term hyperthyroidism
what haematological effects does hyperthyroidism have?
pernicious anaemia, B12 deficiency (only in AITD)
what effect does hyperthyroidism have on reproduction?
oligomenorrhoea, gynecomastia (bc testosterone is developing into oestrogen more rapidly), erectile dysfunction
what is Grave’s disease characterised by?
- Characterised by antibodies against TSHR
* T-lymphocytes commonly seen
why does grave’s disease affect the eyes?
Retroorbital fibrocytes express TSH-receptor
what are risk factors for Grave’s disease?
HLA status (imp bc it’s an autoimmune disease), infection, stress, female
how does Grave’s disease present in men?
Men normally have a severe phenotype that’s harder to treat with anti-thyroid drugs
what are other causes of thyrotoxicosis?
toxic multinodular goitre
toxic adenoma
Excess iodine, Amiodarone, HCG, Thyroiditis, Struma ovarii, TSHoma, Hamburger thyrotoxicosis
how does TSHR cause thyrotoxicosis?
- TSHR in one cell mutates and becomes constitutively active
- Stimulating the TSHR causes hyperplasia so the cell divides
- Mutated TSHR divides
how do you diagnose a toxic adenoma?
Technetium or iodine uptake scans
how do you treat a toxic adenoma?
o Thionamide drugs – Propylthiouracil or Carbimazole
Start with 18 months of C then stop
If it doesn’t work then they either need long term C or different more definitive treatment
o Radioactive Iodine I-131
o Thyroidectomy
how effect is Carbimazole?
Cures 50% of women and 20% of men
what effect does hypothyroidism have on the skin?
- Skin becomes rlly dry
- Hair becomes dry
- Myxoedema in the skin
- Hyaluronic acid accumulation
- Hypercarotenaemia
- Reduced sweat and sebum
- Wounds heal slowly
- Capillary fragility
what effect does hypothyroidism have on the cardiovascular system?
- Reduced cutaneous ciculation = sensitivity to cold
- Sinus bradycardia
- LDL cholesterol up
- J waves of hypothermia
what effect does hypothyroidism have on the GI tract?
reduced appetite and constipation (bc reduced peristalsis)
what effect does hypothyroidism have on nervous system. muscle and bone?
- Impaired fetal brain development
- Dementia
- Slow relaxing reflexes
- Growth retardation
what effect does hypothyroidism have on the renal system?
reduced GFR, mild hyponatremia
what haematological effects does hypothyroidism have?
normochromic normocytic anaemia
• decreased red cell mass and less EPO norm norm anaemia
• B12 def macrocytic
• menorrhagia and achloryhydria - microcytic
what endocrine effects does hypothyroidism have?
delayed puberty, reduced libido, erectile dysfunction
what effects does hypothyroidism have on metabolism?
reduced basal metabolic rate, decreased GLUT4 stimulation
what are the causes of hypothyroidism?
• Hashimoto’s disease; Endemic goitre; Lithium; Cabbage; Infiltrative diseases; Pendred’s syndrome; Hypopituitarism
