Hypothyroidism Flashcards
Which is the active and inactive thyroid hormone
T4 is inactive, T3 is active.
What does T3 basically control
T3 controls basal metabolic rate (body temperature
Describe the hypothalamo-pituitary-thyroid axis inhibitions/stimulations
TSH switches on the follicular cells in the thyroid gland and you release thyroxine into your blood stream. T4/T3 has a negative feedback on the pituitary and hypothalamus so level of TRH and TSH goes down as thyroxine goes up.
What is thyroxine
T4
What stimulates TSH production
The hypothalamus secretes TRH which stimulates the pituitary to make TSH.
What does TSH do molecularly
TSH travels to the thyroid in the blood and tells the thyroid to turn on the trapping of iodide in the blood from the diet.
Describe the synthesis of thyroxine
The hypothalamus secretes TRH which stimulates the pituitary to make TSH. TSH travels to the thyroid in the blood and tells the thyroid to turn on the trapping of iodide in the blood from the diet. The iodide goes into the thyroid follicle cells and is converted to iodine – after this it binds to tyrosine which then converts it to thyroxine.
What cells make thyroxine
follicular cells
How long is our store of thyroxine
A month
usual reason for thyroid failure?
due to autoimmune damage to the thyroid.
What reaction converts T4 to T3
deiodination by deiodinase enzymes
Proportions of circulating T3 from deiodination vs direct thyroid synthesis?
80% from deiodination of T4
20% from direct thyroidal secretion
What is used to treat hypothyroidism
Thyroxine usually given as a salt LEVOTHYROXINE SODIUM; thyroxine sodium; thyroxine; tetraiodothyronine; T4
and sometimes Liothyronine sodium (T3)
What causes primary hypothyroidism
autoimmune, iatrogenic Post-thyroidectomy or post-radioactive iodine
What causes secondary hypothyroidism
pituitary tumour, post-pituitary surgery or radiotherapy
RoA of levothyroxine sodium?
Oral
What do you use to guide levothyroxine sodium dose in primary hypothyroidism
- TSH levels
What do you use to guide levothyroxine sodium dose in secondary hypothyroidism
- TSH low due to anterior pituitary failure Can’t use TSH to guide dose Aim for fT4 middle of reference range
What is administered in primary hypothyroidism
LEVOTHYROXINE SODIUM
What is administered in secondary hypothyroidism
LEVOTHYROXINE SODIUM
When is LEVOTHYROXINE SODIUM used
primary and secondary hypothyroidism
When is LIOTHYRONINE used and why
Myxoedema coma as it is faster and more potent
RoA of LIOTHYRONINE
IV
Adverse effects of thyroid hormone replacement therapy is usually associated with…
low/suppressed TSH
Side effects of thyroid hormone replacement therapy? (4)
- SKELETAL Increased bone turnover, reduction in bone mineral density, risk of osteoporosis
- CARDIAC Tachycardia, risk of dysrhythmia- particularly atrial fibrillation
- METABOLIC Increased energy expenditure, weight loss
- INCREASED BETA-ADRENERGIC SENSITIVITY Tremor, nervousness
Where is circulating thyroxine/thyronine found?
Plasma proteins, mainly thyroxine binding globulin TBG
What must be bore in mind when administering thyroxine/thyronine re. protein binding
- Plasma binding proteins increase in PREGNANCY and on prolongued treatment with OESTROGENS or PHENOTHIAZINES
- TBG falls with malnutrition, liver disease and certain drug treaments
- Certain co-administered drugs (e.g. phenytoin, salicylates) compete for protein binding sites
meaning there will be more bioactive/unbound thyroxine/thyronine
Symptoms of hypothyroidism? (7)
- Deepening voice
- Depression and tiredness
- Cold intolerance
- Weight gain with reduced appetite
- Constipation
- Bradycardia
- Eventual myxoedema coma
hypothyroidism will eventually lead to…
myxoedema coma
What is hypothyroidism known as
Myxoedema
