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
Explain the pathophysiology of Graves
Autoimmune
TSH receptor antibodies stimulate TSH R on the thyroid
This causes excess T4 production
What test allows to see a single toxic adenoma?
A techtenium scan
What are other less common causes of hyperthyroidism?
Viral thyroiditis
Post partum thyroiditis
What causes Post partum thyroiditis ?
Pregnancy
The body produces antibodies that stimulate the thyroid to release excess amounts of thyroxine
What is the technetium scan like for patients with viral /post partum thyroiditis?
LOW uptake on technetium scan
What are clinical features of hyperthyroidism’
Increased metabolic rate CV: tachycardia, palpitations GI: diarrhoea Respiratory: tachypnoea Skeletal: osteopenia Reproductive: irregular periods
How do you investigate primary hyperthyroidism?
Low TSH
High T4
Technetium scan (high/low uptake)
Thyroid antibodies (Graves)
What tests are required once hyperthyroidism is confirmed?
ECG (check for AF) DEXA scan (osteopoenia)
How do you manage hyperthyroidism?
Beta blocker - if pulse >100bpm
Radioactive iodine (taken up by thyroid, releases radiation, destroys thyroid gland)
Thionamides e.g. carbimazole, propylthiouracil
How do thionamides work?
They block Thyroid Peroxidase
Thereby they inhibit the conversion of iodide to iodine
What is a dangerous side effect of thionamides?
AGRANULOCYTOSIS
What are indications for perchlorate
Prior to thyroid surgery in hyperthyroidism
What is the MOA of perchlorate?
It blocks the uptake of iodide by the thyroid cells
What are the types of thyroid carcinoma?
Papillary / follicular thyroid cancer
What is the aggressiveness of thyroid carcinoma?
Indolent (patients survive 40-50 years)
What is the treatment for thyroid carcinoma?
Total thyroidectomy
Then radioactive tx to remove any remaining cells
Then give thyroxine in high doses (this lowers TSH levels, so TSH does not stimulate any remaining thyroid cell cancers)
What is a tumour marker to see if thyroid carcinoma has returned?
Thyroglobulin
What cells are affected by medullary carcinoma of the thyroid?
C cells (which produce calcitonin)
What are tumour markers for medullary carcinoma of the thyroid?
Calcitonin
CEA
