Thyroid Flashcards
What hormone controls uptake of iodide into the thyroid?
TSH
What occurs once iodide enters the thyroid?
It is converted from iodide to iodine by thyroid peroxidase
Explain T4 status possibilities once secreted by the thyroid gland into circulation’
75% is Thyroxine Binding Globulin Bound
20% is TBPA bound
5% is albumin bound
0.03% is FREE T4
Explain the hypothalamus-pituitary-thyroid axis
Hypothalamus produces TRH
TRH stimulates the pituitary to produce TSH
TSH stimulates the thyroid to produce T4
T4 is converted to T3 in the peripheries
Excess T4 negatively feedbacks to the hypothalamus to prevent it from producing too much TRH and the pituitary to produce too much TSH
What occurs to TSH and TRH if you have low T4?
TSH and TRH increase
What are the three main causes of hypothyroidism?
Hashimotos (Autoimmune)
Atrophic thyroid (congenital, with age)
Post-Graves disease (due to radioactive iodine, surgery, thionamines)
What are other causes of hypothyroidism?
Post-thyroiditis Drugs (amiodarone, lithium) Thyroid agenesis/disgenesis Iodine deficiency Secondary hypothyroidism
What are clinical features of hypothyroidism?
Slow metabolic rate - tiredness, weight gain, poor appetite Cardiovascular - bradycardia GI - constipation Respiratory - laboured, slow breathing Reproductive - oligomenorrhoea Cold hands and feet Hyponatraemia Normocytic anaemia Myxoedema, goitre
How does hypothyroidism cause hyponatraemia’
Low thyroxine causes low cardiac output
Low CO increases ADH production
ADH increases water reabsorption (not salt)
Hence hyponatraemia occurs
What investigations should you perform + findings to confirm hypothyroidism?
High TSH + low fT4 in primary hypothyroidism
Thyroid peroxidase antibodies (autoimmune)
What antibodies confirm Hashimoto thyroiditis?
TPO (thyroid peroxidase) autoantibodies
What investigation is fundamental before starting a patient on thyroxine treatment?
Do an ECG
If patient has CV disease, giving them thyroxine will increase cardiac contractility, causing ischaemia
What medication can you give for primary hypothyroidism?
Levothyroxine (T4)
What are risks of excess levothyroxine?
Osteopoenia
AF
When should you prescribe T3?
Never
Because there is no evidence that it is beneficial over T4
What does the blood test look life for subclinical hypothyroidism?
T4 levels are NORMAL
TSH levels are HIGH
essentially COMPENSATED hypothyroidism
What is the benefit of treating subclinical hypothyroidism?
That SH associated with hypercholesterolaemia, treating it will also treat the hypercholesterolaemia
What is an important sequela of radioactive treatment?
HYPOTHYROIDISM (usually occurs within 1 year)
Explain how thyroid function changes in pregnancy
hCG rises in first trimester
hCG acts similarly to TSH: it stimulates the thyroid gland to produce thyroxine > rise in T4
Oestrogen triggers increase of TGB
How is neonatal hypothyroidism diagnosed?
Gunthie test
Measures TSH >48 h after birth (if before that, will measure mothers TSH not baby’s)
What occurs to thyroxine levels in a very sick patient?
They are SICK EUTHYROID
Normal / high TSH with low T4
This is because the thyroid sits down to reduce metabolioc rate
How do you treat sick euthyroid patients?
You don’t
They do not have hypothyroid symptoms
What is the typical blood results of someone with primary hyperthyroidism?
High T4
Low TSH
What are 3 common causes of primary hyperthyroidism?
Graves disease
Toxic multi nodular goitre
Single toxic adenoma
