5. Hyperthyroidism Flashcards
What are the 2 common causes of hyperthyroidism?
- Grave’s disease
* Plummer’s disease
What is Grave’s disease?
- Autoimmune disease
- Antibodies bind to and stimulate the TSH receptor in the thyroid
- Thyroid becomes overactive as a result
- Becomes smoothly enlarged (goitre)
- Hyperthyroidism
What does a typical Grave’s patient present with?
- Lid lag - delay in moving eyelid as the eye moves down
- Overactive, anxious, too much energy, sweating
- Rapid pulse (from raised BMR)
- Warm
- Localised pretibial myxoedema
- Exophthalmos
What can make exophthalmos worse?
Smoking
What causes pretibial myoedema and exopthalmos?
- Pretibial myxoedema - binding of a different antibody to receptors in the soft tissue of the shin => growth
- Exopthalmos - binding of another antibody to growth factor receptors behind the eye, so muscles behind grow and push eyes forward
(3 different antibodies cause hyperthyroidism, PM and exopthalmos)
How can you image the thyroid and how would you detect a tumour in Grave’s?
• Scintigraphy
• Administer radioactive iodine
• Thyroid is the only part of the body that takes up iodine
• Image shows where iodine is present
- normally, whole thyroid seen
- tumour, “cold nodule” with no iodine used
What is Plummer’s disease?
- Toxic nodular goitre
- Not autoimmune
- Benign adenoma
- Overactive - producing thyroxine
- No pretibial myxoedema or exophthalmos
- Patients may have a multinodular goitre
How can you test for Plummer’s disease?
- Lump on one side - tumour just on one part of thyroid
- Technetium or iodine scan - iodine will go into the tumour, “hot nodule”
- Normal part of thyroid shrinks and stops making thyroxine ∵ tumour produces too much thyroxine => pituitary stops making TSH
What are the effects of thyroxine on the sympathetic nervous system?
• Sensitises beta adrenoceptors to ambient levels of adrenaline and NA
- normal levels of adrenaline and NA will thereforenhave much stronger effects
• Too much adrenaline - palpitations, tachycardia, tremor in hands, lid lag (muscles that open the eye are half sympathetic and half oculomotor)
What is a thyroid storm (thyrotoxic crisis) and how does it present in a patient?
- Severe complication of hyperthyroidism
- Hyperpyrexia (>41’C)
- Accelerated tachycardia/arrhythmia - >170bpm
- Cardiac failure
- Delirium/frank psychosis
- Hepatocellular dysfunction, jaundice
- High risk of death if hyperthyroidism has been neglected and presented with at least 2 of the above
- 50% mortality if untreated
How is a thyroid storm treated?
Prompt and aggressive treatment:
• surgery
• radioiodine
• drugs
What classes of drugs can be used to treat hyperthyroidism?
• Thionamides • Potassium iodide • Radioiodine (above reduce thyroxine synthesis) • Beta-blockers (helps reduce symptoms)
When are thionamides e.g. (propylthiouracil and carbimazole) used?
- Daily treatment of hyperthyroid conditions
- Control hyperthyroidism before thyroidectomy (so general anaesthetic isn’t give to someone who is tachycardic etc.)
- Following radioactive iodine treatment - which takes a while to work
Summarise the synthesis of thyroid hormones
- Iodine taken up into follicular cells
- Iodination of tyrosine residues in thyroglobulin - using thyroperoxidase + hydrogen peroxide (in the colloid)
- Coupling of monoiodotyrosine and diiodotyrosine to form T3 and T4 (in the colloid)
- Taken up then released from cells into circulation
Describe the mechanism of thionamides
• Inhibit thyroperoxidase
• Therefore, inhibit iodination of thyroglobulin and the coupling of iodotyrosines
• Reduction in synthesis and secretion of thyroid hormones
• Biochemical effects within hours, clinical effects take weeks
- lots of stored thyroid hormone in lumen
- T4 has long half-life
• May also suppress antibody production in Graves’ disease
• Propylthiouracil reduces deiodination of T4 to T3 in peripheral tissues
