S2: Thyroid Gland and it's Disorder Flashcards
Anatomy of thyroid gland
The thyroid gland is a butterfly shaped gland, in the neck over the trachea. It consists of a right and left lobe joined by a narrow isthmus. There is also a variable pyramidal lobe. It weighs around 15-20 grams and there are parathyroid glands at the back.
What two hormones does the thyroid gland produce?
T4 – thyroxine
T3 – Triiodothyronine
Explain how the thyroid gland is under control from the hypothalamo-pituitary axis
So hypothalamus releases TRH (Thyrotropin releasing hormones), which stimulates the pituitary gland to release TSH (thyroid stimulating hormone), which stimulates the thyroid gland to synthesise and release T3/T4.
There can then be negative feedback of TH on pituitary gland and hypothalamus
Why does thyroid gland mostly release T4?
The thyroid gland mostly releases T4, but T3 is the much more active one. Therefore T4 may be converted to T3 in the liver but also specifically in target tissues (through the deiodinases), because your heart will need different amount of T3 compared to your CNS/brain etc.
What are the two cells thyroid gland is made up of?
- Thyroid follicles (follicular cells + colloid). Follicular cells produce thyroid hormone.
- Para-follicular cells (C cells) that produce calcitonin which are involved in calcium metabolism.
What does the colloid contain?
Contains thyroglobulin which is a protein synthesised by the follicular cells. Thyroglobulin contains tyrosine residues, which get combined with iodine which allow the formation of thyroid hormones.
How many iodines do T4 and T3 contain?
T4 – contains 4 iodines
T3 – contains 3 iodines
Is reverse T3 active?
No it is inactive
What is T3 and T4 made of?
Tyrosine + Iodine = monoiodotyrosine (MIT)
Monoiodotyrosine + iodine = diiodotyrosine (DIT)
MIT + DIT = T3
DIT + DIT = T4
How do circulating thyroid hormones travel in the blood?
Over 99% of circulating thyroid hormones are bound to plasma proteins of which
- 70% is bound to thyroid binding globulin
- 10-15% bound to transthyretin
- 20-15% bound to albumin
Only a very tiny fraction is in the free form
Two types of goitre
Enlargement of the thyroid gland is called goitre
- If it is uniformly swollen it is called diffuse goitre. - If it is lumpy, it is nodular goitre.
Metabolism in hyperthyroidism and hypothyroidism
Hyperthyroidism – Too much thyroid hormone, metabolism speeds up
Hypothyroidism – Too little thyroid hormone, metabolism slows down
Symptoms of Hyperthyroidism
- Weight loss despite increased appetite
- Anxiety
- Palpatations
- Diarrhoea
- Hyperactivity
- Tremor
- Eye signs
- Tachycardia
Exopthalamos (bulging eyes)
Which gender is hyperthyroidism more common in?
Women
Causes of hyperthyroidism
Commonly it is autoimmune, in which case it would be Grave’s disease, or toxic multinodular goitre.
Uncommon causes are toxic adenoma or thyroiditis.
Rare is TSHoma or follicular thyroid cancer.
Symptoms of Grave’s
- Smooth goitre and exopthalamos
More eye signs include:
- Periorbital oedema
- Chemosis (swelling)
- Lid retraction/lid lag
- Proptosis (exophthalmos)
- Opthalmoplegia (paralysis)
Other features of Graves include: Diabetes, Vitilligo, Onycholysis, Dermopathy, Acropachy - This is because Grave’s disease is an autoimmune disease, so others are common
How is primary hyperthyroidism diagnosed?
- Suppressed TSH
- Elevated thyroid hormones
- In patients with overt opthalmopathy, no additional testing is required for the diagnosis
- Isotope uptake studies can be used
Why is it hard to diagnose hyperthyroidism in the elderly?
- Classical signs and symptoms may be lacking
- Goitre may be absent
- Anorexia with wasting
- AF or congestive heart failure may be predominant manifestations
Treatment of Hyperthyroidism
- Antithyroid medications can be used including Carbimazole, Methimazole and Propylthiouracil block TPO (thyroid perioxidase) enzyme.
- Beta blockers will help patients feel better, as the effects of high thyroid are like there is high adrenaline, note it is not the treatment but temporary relief while other medications work.
- You can use radioactive iodine as well as surgery
What is a thyroid storm?
Thyroid Storm is an extreme form of thyrotoxicosis and occurs with stressful illness or thyroid surgery. It is manifested by marked delirium, severe tachycardia, vomiting, diarrhoea, dehydration and very high fever.
Mortality rate is high in this state.
Difference primary and secondary hypothyroidism
Hypothyroidism is very common, there is then the distinction between primary (caused by underfunction of thyroid gland itself) or secondary (due to pituitary gland underfunctioning).
Symptoms of hypothyroidism
- Fatigue
- Cold intolerance
- Weight gain
- Bradycardia
- Constipation
What is Hashimoto’s disease?
Hashimotos disease is the most common cause of hypothyroidism in iodine sufficient areas and is due to antibodies again directed towards TPO or thyroglobulin.
It is also much more common in females. Hashimotos is a form of primary hypothyroidism.
Causes of Hypothyroidism
- Hashimoto’s disease
- Post-thyroidectomy (someone had an overactive thyroid and you removed it)
- Post RAI (radioactive iodine, destroys it).
- There is also iodine deficiency or iodine excess, as well as certain drugs and secondary hypothyroidism.
Investigations for hypothyroidism
We would note low free T4 and high TSH (if primary)
Could test TPO antibody levels
Treatment for hypothyroidism
Treatment is often managed by the GP, by giving them the missing hormone in the form of levothyroxine.
What does it suggest if patient taking thyroxine (for hypothyroidism) feels worse?
Patients still can develop other autoimmune conditions (e.g. addisons, if you put a person on thyroxine and they feel worse, it is likely they have addisons disease)
What is a myxoedema coma?
This is the opposite of a thyroid storm.
Could be caused by long-standing untreated hypothyroidism, precipitated by infection, myocardial infaction, cold exposure or sedative drugs.
Mortality rate is 20-25%
Results in decreased mental status, hypothermia and other symptoms related to slowing of function in multiple organs.
List different types of thyroiditis
Hashimotos thyroiditis -> painless, diffuse goiter
Post-partum thyroiditis (after delivery of baby)
Subacute thyroiditis -> Anyone can get, painful thyroiditis
Acute thyroiditis -> Bacteria, fungus etc
Consequence of goitre
- Dysphagia = difficulty swallowing
- Dysphonia = Inability to produce sounds
- Dyspnoea = Breathless
List types of thyroid cancer
Papillary Thyroid carcinoma
Follicular carcinoma
Anaplastic carcinoma (
List of investigations undertaken in someone has a thyroid lump
- Find out symptoms match hyper or hypo
- Decide if diffuse or nodular goitre
- Do thyroid blood test, are they eu-,hypo-hyper- thyroid?
FNA (fine needle aspiration) cytology, to have a look at it and take a few cells to look at under a microscope
