Thyroid Drugs Flashcards
Basic Thyroid Physiology Mechanisms:
TRH is released by the hypothalamus, which acts on the thyrotrophs in the anterior pituitary to release TSH. TSH directly binds Thyroid Receptors to stimulate thyroid production (T3 and T4)
- T4 exerts negative feedback inhibition of TSH release
- TSH inhibits TRH release from the hypothalamus.
Thyroid Physiology:
Iodide enters the thyroid where it is converted to Iodine by Thyroid Peroxidase.
- Once converted into Iodine, Thyroid Peroxidase is also responsible for adding the iodine to tyrosine (a process known as Iodination) and facilitates coupling of MITs and DITs which form T3 and T4.
- T4 is more abundantly synthesized, however, T3 is more active.
- T4 can undergo conversion to T3 and rT3 (inactive).
Thyroid Receptors:
TR-alpha and TR-beta
- Once bound by TH, the thyroid receptor can combine in several different ways.
- -Can combine with each other to form a homodimer
- -Can combine with other transcription factors to form a heterodimer.
- Either way, this promotes gene expression (which can have different effects within the tissues)
- ->TH has effects on growth and development in the CNS.
*TH has effects on metabolism and energy expenditure.
Characteristics of Hyperthyroidism:
-Excess energy (anxiety)
-Heat intolerance
-Weight loss
(Tachycardia & sweating)
Characteristics of Hypothyroidism:
-Low energy (depression)
-Cold intolerance
-Weight gain
(Bradycardia & Dry Skin)
What is Graves Disease?
Thyroid Stimulating Immunoglobulin (Tslg) is produced, which binds TSH receptors to stimulate production of thyroid hormone.
-No negative feedback so there is continual production of TRH.
–Leads to hyperthyroidism
-Treated with drugs that reduce TH release.
(In graves disease, TSH will be low)
What is Hashimoto’s Thyroiditis?
Gradual inflammation and destruction of the thyroid gland.
-After initial flare of TH hormone release, TH becomes low.
-Leads to Hypothyroidism.
–Treated with TH supplementation.
(In Hashimoto’s disease, TSH will be too high)
Levothyroxine
T4 supplementation
-Converted to T3 within the body.
*The preferred option for hypothyroidism.
T1/2= 7 days so there is less fluctuation in T3 levels
(Administering T4 may provide a “reservoir” of TH that can be converted to T3 when needed.
- Should be taken at a consistent time, daily.
- Best taken on an empty stomach.
- Most DDI’s with Ca2+, Mg2+ and Al3+
Adverse Effects (Similar to hyperthyroidism):
-Tremor, anxiety, diarrhea, reduction in bone mineral density are all common adverse effects.
Serious AE:
-Cardiac Effects (most concerning–tachycardia & atrial fibrillation)
Liothyronine:
- T3 supplementation
- Shorter half-life, so there are more fluctuations in levels.
- Increased risk of CV adverse effects.
–>Possible alternative if levothyroxine isn’t tolerated… however, levothyroxine is preferred.
What happens when combining T3 and T4 supplementation?
Desiccated Thyroid hormone is an older product with T3 and T4 (Porcine source)
*Quite unreliable.
‘Natural’ Thyroid Supplementation:
Natural Health Products (NHP’s) for thyroid are available in Canada without a prescription.
-Many have not even thoroughly studied by Health Canada.
Methimazole/ Propylthiouracil (PTU):
Thyroid Peroxidase Inhibitors (Thionamides)
- Inhibit various stages in thyroid hormone synthesis
- PTU also inhibits the peripheral conversion of T4 to T3
- Better for managing thyroid storm (very high TH levels)
- Thionamides inhibit thyroid hormone synthesis via inhibition of Thyroid Peroxidase (inhibits iodination and coupling of MIT and DIT)
Adverse Effects:
- Caution using pregnancy (especially in the 1st Trimester)
- Rash
- Hypothyroid-like symptoms if too much is taken.
- Serious/ rare AE’s include agranulocytosis, Anti-Psychotic effects and Hepatotoxicity.
Iodine 131
Radioactive Iodine
- Taken up by the thyroid gland which causes the thyroid gland to atrophy.
- Contraindicated in pregnancy.
Adverse Effects:
-Hypothyroidism
(The gland is destroyed, so patients will likely require supplementation)
-Relatively safe option.
-CAN cause damage to the salivary glands (Dry Mouth)
-Increased risk of Tooth Decay
-Altered Taste Sensation
-Pain
-Patients remain radioactive for a few days (danger to children)
Iodide Supplementation Options:
-Saturated Solution of Potassium Iodide (SSKI)
-Lugol’s Solution (Iodine/ Potassium Iodide)
–>Iodide is taken up by the Thyroid Gland & inhibits TH synthesis/ release.
(Wolf- Chaikoff effect)
*Old time solutions are not used anymore.
If Tachycardia accompanies hyperthyroidism, what drug should be prescribed to treat this adverse effect?
Beta Blockers!
