ENDOCRINOLOGY - HYPERTHYROIDISM Flashcards
What is thyrotoxicosis?
The clinical syndrome of excess circulating thyroid hormones, irrespective of the source.
Outline the basic structure of thyroid anatomy?
Comprises 2 lobes connected by an isthmus and they extend from the oblique line on thyroid cartilage to the 5th-6th tracheal rim.
What’s the histology of the thyroid gland?
The functional unit is the follicle - this consists of epithelial follicular cells surrounding a colloid
The thyroid also contains parafollicular cells which produce calcitonin
Outline the synthesis and release of thyroid hormones?
Follicular epithelial cells secrete thyroglobulin into the colloid. Binding of TSH to its receptors in the follicle cells of the thyroid gland causes the cells to actively transport iodide ions across their cell basolateral membrane from the bloodstream via a sodium iodide cotransporter (2NA+ and I-) - Na+ transported back into blood via Na+K+ ATPase. Iodide ions are then transported across the apical membrane into the colloid space via the pendrin cotranporter (I- in return for Cl-)
In the colloid space, thyroperoxidase oxidises 2 iodide ions to form 1 iodine molecule.
thyroperoxidase enzymes iodinates tyrosine residues in thyroglobulin to form T4, T3 and r-T3
These hormones remain in the colloid centre of the thyroid follicles until TSH stimulates endocytosis of colloid back into the follicle cells. There, lysosomal enzymes break apart the thyroglobulin colloid, releasing T3 and T4 so they can diffuse across the follicle membrane and enter the bloodstream.
How are T3 and T4 transported in the blood?
Less than 1% travels unbound and this free T3 and T4 cash cross the lipid bilateral of cell membranes.
The remaining 99% is bound to thyroxine binding globulins, albumin or other plasma proteins. This prevents their free diffusion into body cells. When blood levels of T3 and T4 begin to decline, the bound molecules are released from the plasma cells and can readily cross membranes of target cells
What’s the blood supply to the thyroid gland?
Superior (from ECA) and inferior thyroid artery (from thyrocervical trunk)
What’s the venous drainage from the thyroid gland?
Superior, middle and inferior thyroid veins which form a venous plexus around the thyroid gland
Superior and middle veins drain into the internal jugular vein
Inferior vein drains into the brachiocephalic vein
Outline the HPA axis for thyroid?
Thyrotrophin releasing hormone is secreted in the hypothalamus and travels via the portal system to the anterior pituitary where is stimulates thyroid stimulating hormone
TSh is secreted into the systemic circulation where it stimulates increased iodine uptake by the thyroid, and synthesis and release of T4 and T3
TSH also stimulates the conversion of T4 to T3 (more active hormone) in peripheral tissues
T3 and T4 enter cells where they bind to nuclear receptors and promote increased metabolic and cellular activity
If T3 and T4 rise above normal levels, TRH and TSH production is suppressed so peripheral T3 and T4 levels can fall to normal.
Where peripherally is T3 produced from T4?
Liver, kidney and spleen
Why is T4 converted to T3 in peripheral tissues?
Because more T4 is produced but T3 is 5x more bio active
What are the main causes of thyrotoxicosis with hyperthyroidism?
Graves’ disease
Toxic multinodular goitre
Solitary toxic adenoma
Amiodarone induced thyrotoxicosis
Follicular thyroid cancer
Beta-HCG related e.g. in pregnancy or hydatidiform mole
Secondary hyperthyroidism caused by a TSH-secreting pituitary adenoma
What’s the difference between primary and secondary hyperthyroidism?
Primary - when pathology is within the thyroid gland
Secondary - where the thyroid is producing excessive thyroid hormone as a result of overstimulation by thyroid stimulating hormone. The pathology is in the hypothalamus or pituitary.
What are the main causes of secondary hyperthyroidism?
TSH secreting pituitary adenoma
Thyroid hormone-resistance syndrome
HCG secreting tumour
Gestational thyrotoxicosis (excessive stimulation of thyroid gland by HCG)
What are the main causes of hyperthyroidism?
Graves’ disease - 75%
Toxic modular goitre
Thyroiditis
Drugs
What is Graves’ disease
an autoimmune condition where TSH receptor antibodies cause a primary hyperthyroidism. Serum IgG antibodies bind to TSH receptors in the thyroid, stimulating thyroid hormone production
What is graves opthalmopathy?
Aka thyroid eye disease
Bulging of eyeball out of the socket due to inflammation, swelling and hypertrophy of the tissue behind the eyeball (exophthalmos)
Smoking significantly increases the risk
SEE GP PROMPTLY IF NOTICE PAIN< REDNESS< SWELLING, bulging eyes, blurred or double vision
What is pretibial myxoedema?
Dermatological condition where there are deposits of mucin under the skin in the pre-tibial area of the leg; this gives a discoloured, waxy, oedematous appearance to the skin over this area
This is specific to Graves’ disease
What are some unique features of Graves’ disease?
Graves eye disease
Pretibial myxoedema
Thyroid acropachy
What are some universal features of hyperthyroidism?
Anxiety and irritability
Sweating and heat intolerance
Tachycardia
Weight loss despite eating normal/more
Fatigue
Diarrhoea
Sexual dysfunction
Weakness
Tremor
Palpitations
Increased persiration
What are risk factors for hyperthyroidism?
Family history
High iodine intake
Smoking (esp for Graves’ disease)
Female
Co-existent autoimmune disease
Trauma to thyroid gland
Childbirth
What are features unique to multinodular goitre?
Goitre with firm nodules (nodules are often present years before thyrotoxicosis occurs)
Most patients are aged over 50
What is thyroiditis?
Inflammation of the thyroid gland. Destruction of thyroid cells causing release of thyroid hormone into the blood - can be caused by viruses and drugs
What drugs commonly cause hyperthyroidism?
Lithium and amiodarone
Overdose of levothyroxine
Outline how amiodarone can affect the thyroid function?
Amiodarone-induced thyrotoxicosis. It can cause hypothyroidism and, less commonly, hyperthyroidism
Amiodarone is a heavily iodinated drug that blocked the conversion of T4 to T3. This leads to a decrease in inhibition at pituitary due to low levels of T3 = increase TSH levels. Overtime increased TSH causes increased T4 production and eventually the higher levels of T4 can overcome the blockade of amiodarone and convert to adequate levels of T3. This increases inhibition at pituitary level. After several months, TSH is normal with high T4 and low T3.
Patients should have their thyroid function tested before going on amiodarone
This is more common in areas with low iodine intake
What are the 2 types of amiodarone induced thyrotoxicosis?
Type 1 - thyrotoxicosis in a patient who has underlying thyroid dysfunction.
Type 2 - thyrotoxicosis resulting from destructive thyroiditis in which there is excess release of pre-formed T4 and T3 into the circulation. It typically occurs in patients without underlying thyroid disease and is caused by a direct toxic effect of amiodarone on thyroid follicular cells. The thyrotoxic phase may last several weeks to several months and it is often followed by a hypothyroid phase with eventual recovery in most patients
Which demographic does Graves’ disease typically affect?
Younger aged women (F:M 10:1)
Often familial
What demographic does nodular thyroid disease typically affect?
Older age
Females/males equally
Rarely familial
What does the presence of a multi nodular goitre without Graves’ disease symptoms suggest?
Toxic nodular goitre
Why is thyrotoxic crisis?
An extreme manifestation of thyrotoxicosis due to overproduction of thyroid hormones
It occurs as a sudden onset of hyperpyrexia (>41 degrees). Tachycardia
Diarrhoea and nausea
Jaundice
Severe agitation and confusion
Loss of consciousness
What are the clinical symptoms and signs of thyrotoxicosis?
Anxiety
Emotional lability
Proximal myopathy
Fine tremor
Palpitations
Heat intolerance
Increased perspiration
Weight loss despite normal or increased appetite
Agitation
Lid retraction and lag
Orbital inflammation
Warm and moist skin
Tachycardia or AF
Goitre
Palmar erythema
Hair thinning
Brisk reflexes
oligomenorrhoea or amenorrhoea
What is thyrotoxic periodic paralysis?
A serious complication characterised by muscle paralysis and hypokalaemia due to a massive intracellular shift of potassium
Acute proximal symmetrical lower limb weakness
How do you investigate hyperthyroidism?
TFTs: serum TSh should be suppressed and T3 and T4 raised
Check for TSH receptor antiboides
Can ultrasound for goitre or MRI of orbits
thyroid uptake scans
What do compression symptoms of breathlessness, hoarse voice, dysphasia and neck pressure suggest with hyperthyroidism?
A large enough goitre - typically toxic multi nodular goitre
How does a goitre present in Graves’ disease?
Diffusely symmetrically enlarged without nodules and there may be a bruit
How does a goitre present in toxic multinodular goitre?
Non-tender thyroid nodules
How does a goitre present in toxic adenoma?
Unilateral, non-tender thyroid made
How does a goitre present in subacute thyroiditis?
Tender, firm, irregular, diffusely enlarged thyroid gland which may be asymmetrical
How does a goitre present in amiodarone-induced thyroiditis?
Small goitre usually present
How to investigate hyperthyroidism?
Check serum TSH levels
If TSH level is low then T3 and T4 should be measured
Suspect overt hyperthyroidism if TSH level is low and T4/T3 raised
Suspect SU clinical hyperthyroidism if TSH is below normal and T3 and T4 are normal
Consider checking TSh receptor antibodies if Graves’ disease is suspected
Measure thyroid peroxidase antibodies if woman is postpartum and postpartum thyroiditis is suspected
Check inflammatory markers if a diagnosis of thyroiditis is suspected (CRP and ESR)
How to manage hyperthyroidism?
Anti thyroid drugs
Radioactive iodine treatment
Thyroid surgery
What are the 2 anti thyroid drug treatments?
Carbimazole and propylthiouracil - act as a preferred substrate for iodisation by thyroid peroxidase
Which anti thyroid drug treatment is used first line and why?
Carbimazole because propylthiouracil has a small risk of severe liver injury
How does radioactive iodine treatment work for hyperthyroidism?
It gets taken up by the thyroid and then uses beta and alpha rays to induce damage of DNA leading to death of thyroid cells, causing a decrease in thyroid function/reduction in thyroid size
Who is radioactive iodine treatment indicated first line in?
First line treatment for adults with Graves’ disease and toxic multinodular goitre