Thyroid diseases Flashcards
What are the agents available for hypothyroidism?
Levothyroxine
Liothyronine
What are the agents available for hyperthyroidism?
Thionamides: Carbimazole, Propylthiouracil (PTU)
Iodides
Non-selective BB
Describe the MOA of levothyroxine.
Exogenous, optically active levo isomer of thyroxine (T4)
Describe the MOA of liothyronine.
Exogenous suppl of T3
Describe the MOA of carbimazole.
Competitively inhib thyroperoxidase –> inhib iodination of TH
Decreased TH synth
Describe the MOA of PTU.
Competitively inhib thyroperoxidase –> inhib iodination of TH
Decreased TH synth
Inhib conversion of T4 –> T3 in periphery at high dose
Describe the MOA of iodides.
At low doses iodides stim uptake
At high doses inhib TH release
Minimal effect on hormone synth
Decrease vascularity and size of thyroid gland
Describe the MOA of non-selective BB.
Blocks beta adrenergic rece –> bradycardia
Blocks conversion of T4 –> T3 at high doses
Describe the PK of levothyroxine.
A:
- Good PO F, 70-80%
- Onset: 3-5d (6-8h IV)
- Mainly absorbed in duodenum and jejunum but affected by gastric pH
D:
- t1/2: 7h
- Highly plasma protein bound (>99%)
M:
- Hepatic (glucuronidation and sulfation)
- Renally (deiodinated thyroid hormones)
E:
- Primarily renally cleared
- Metabolites excre in urine and faeces
Describe the PK of liothyronine.
t1/2: 1-2.5d
Describe the PK of carbimazole.
A:
- Good PO F
- Meta to methimazole in serum upon absorption
- Onset quicker than methimazole
D:
- t1/2 longer than methimazole
- t1/2 (methimazole): 4-6h (effects last whole day as it concentrates in thyroid gland)
- Methimazole does NOT bind to plasma protein (>90% inhib of thyroid organification of iodine within 12h)
M:
- CYP450 and FMO enzymes
E:
- >90% PO admin carbimazole excre as methimazole and its metabolites
- Rest in faeces (enterohepatic circ)
What are the S/E of levothyroxine and liothyronine?
Cardiac abonormalities (tachyarrhythmias, MI, angina)
Seizures
Tremors
Anxiety
Insomnia
Decreased mineral bone density –> increased fracture risk
Alopecia
Diarrhoea
Liothyronine > levothyroxine
What are the S/E of carbimazole?
Rash, risk of SJS
Agranulocytosis (RARE, early on in thera, usually within first 3mo)
Fever
Joint pains
NV
Congenital malformations
What are the S/E of PTU?
Rash, risk of SJS
Agranulocytosis (RARE, early on in thera, usually within first 3mo)
Fever
Joint pains
NV
Hepatotoxicity
How do we monitor hypothyroidism?
4-8 weeks after ini or titration
TSH:
- Target 0.4-4.0 mIU/L
- Elderly up to 6.9 mIU/L ( age related increase in TSH, controversial)
FT4 in central hypothyroidism
After euthyroid, Q6mo-1y in non-pregnant adult pt
How do we monitor hyperthyroidism?
4-6mo after ini or titration
FT4
T3 may be a better marker of effectiveness in early stages of treatment
TSH may remain suppressed for months after thera begins
What is the recommended dosing regimen for levothyroxine?
Ini: 100 mcg once daily
50-60y with no cardiac risk: 50 mcg once daily
W CVD: 12.5-25 mcg once daily
Titration:
Increase or decrease by 12.5-25 mcg daily or 10-15% of total weekly dose
What is the recommended dosing regimen for carbimazole?
Ini: 15-60 mg daily in two or three divided doses
Once euthyroid: reduce to 5-15 mg once daily
What is the recommended dosing regimen for liothyronine?
Normal: 25 mcg
Elderly and CVD: 5 mcg
When is non-selective BB indicated?
Symptomatic relief of hyperthyroidism
Bridging therapy
- Waiting for surgery, ablation
- Waiting for effects of thionamides
Treatment of thyroiditis
PRN for high risk pts e.g. elderly w CVD
When is carbimazole indicated?
Hyperthyroidism, preferred for Graves
2nd-3rd trimester of pregnancy
(Avoid in 1st trimester)
What are the pregnancy considerations for thionamides?
Carbimazole
1st trimester: CI
2nd-3rd trimester: > PTU
PTU
1st trimester: > carbimazole
2nd-3rd trimester: CI
When are iodides indicated?
Before surgery (7-10d): to shrink thyroid gland
After ablative thera (3-7d): to inhib thyroiditis mediated release of stored TH
Thyroid storm
When is liothyronine indicated?
When T4 and T3 combi considered (TSH normalised but still symptomatic for hypothyroidism)
Myxedema coma (more potent)
What are the special counselling points for thionamides?
Slow onset, maximal effect may take 4-6mo
No effect on existing thyroid hormone stores –> wait for them to deplete
May not req lifelong treatment after resetting thyroid
- Remission rate low: 20-30%
- Remission = Normal TSH & T4 for 1y after discontinuing antithyroid - feedback loop normal
What are the special counselling points for levothyroxine?
Take 30-60min before breakfast or 4h after dinner or other medications
Pay special attention to polyvalent ions: space at least 2h apart (otherwised decreased absorption). Dietary fibre can also cause erratic absorption.
Takes 2-3 weeks for symptomatic relief (-ve feedback)
Takes time for FT4 to normalise (-ve feedback)
