Thyroid Disorders Flashcards
Explain how negative feedback helps to regulate thyroid hormone levels
Hypothalamus release TRH to pituitary. Pituitary in turn then releases TSH to the thyroid gland.
Negative feedback works when free T4 hormones act on both hypothalamus and pituitary to stop it from releasing TRH and TSH.
What are the physiological functions of the thyroid hormones?
Thyroid hormones increases oxygen consumption and thus, increase basal metabolic rate. This then affects body temperature, CNS, sleep, cardiac function, GI function, muscle strengthening, breathing and menses.
Determine the changes in TSH seen for the two main types of thyroid disorders
Hypothyroidism: TSH elevated
- increase due to attempt to unsuccessfully try and stimulate thyroid hormone
Hyperthyroidism: TSH decreased
- due to high TH whereby hypothalamus attempts to decrease high secretion of TH
What is essential for the production of thyroid hormones?
Iodine consumption. Iodine must be obtained exogenously
Define what is thyroxine binding globulin and explain its physiology.
Assuming patient has no thyroid issue, pregnant women and those on estrogen tend to have a higher TBG. As a result, FT4 and FT3 decreases since more T3 and T4 binds to extra TBG. TSH is then released to instruct thyroglobulin to release higher levels of thyroid hormones to return thyroid hormones to equilibrium.
What are the compelling indications for screening thyroid function test?
Symptoms of hyperthyroid/hypothyroid
History of head/ neck radiation for malignancies
Presence of autoimmune disorders
Taking amiodarone/lithium
Psychiatric disorders
First degree relative with autoimmune thyroid disorders
Which subgroup population requires routine screening of thyroid function test?
Pregnant
Pediatric
Define hypothyroidism
Decrease in activity of thyroid gland
What are the primary and secondary causes of hypothyroidism?
Primary causes: Iodine deficiency, Early hashimoto disease (ATgA Antibodies; TPO Autoantibodies); Iatrogenic (due to thyroid resection or RAI)
Secondary causes: Central hypothyroidism (due to hypothalamus or pituitary); drug induced
What are the signs and symptoms associated with hypothyroidism?
Cold intolerance
Dry skin
Fatigue, lethargy, weakness
Weight gain
Bradycardia
Slow reflexes
Coarse skin and hair
Periorbital swelling
Menstrual disturbances: more frequent and more blood
Goiter: thyroid gland enlargement
What are the potential complications listed with those of hypothyroidism?
Increase in LDL and TG, Increase atherosclerosis, Increase CPK, Increased miscarriage risk and impaired fetal development
How do I diagnose one with hypothyroidism?
Check for signs and symptoms
Do a thyroid function test.
Primary hypothyroidism: Low T4 and high TSH
Central hypothyroidism: low T4 and TSH
What drug should I consider initiating for those with hypothyroidism and what is the agent’s mode of action?
Levothyroxine
Provide and replaces synthetic T4
How should I dose patients initiated on levothyroxine? What do I consider for their dosing?
Young healthy adults: 1.6 mcg/kg/day
50-60 years: 50mcg daily
CVD risk: 12.5 -25 mcg . day
How do I titrate patient/s levothyroxine dose?
Increase dose by 12.5 - 25 mcg / day
Increase by 10-15%
What should I advice patients who are taking levothyroxine for their hypothyroidism?
Take 30-60 mins before breakfast OR 4 hours after dinner. Best taken on an empty stomach
Avoid taking with calcium/ iron supplements and antacids
Cite monitoring parameters and frequency of monitoring for those on levothyroxine upon initiation and maintenance dosing.
Monitor 4-8 weeks upon initiation to assess response of TSH.
Upon reaching euthyroid state, thyroid functions test can be done every 6 month-1 year
For patients on central hypothyroidism, what monitoring parameters should I look out for?
FT4
TSH will be inaccurate as it will remain suppressed long term.
What are the adverse drug reactions of levothyroxine?
Increased cardiac abnormalities
Hyperthyroidism
Increases risk of fractures
What is the mode of action of liothyroxine and why is it not recommended ?
Act as synthetic T3 and is not recommended due to short t1/2 and higher susceptibility to ADR
What is the place in therapy for liothyroxine though not widely used?
Normalized TSH with complains of hypothyroidism symptoms
Myxedema coma
For patients who are pregnant and have hypothyroidism, what are some potential risks associated if untreated?
Increased risk of miscarriage and spontaneous abortion
Increased congenital defects
Why should hypothyroidism be treated ASAP for those who are pregnant?
Maternal thyroid hormones provide fetus thyroid hormones for up to 6 weeks
Takes time for fetus to form their own thyroid gland
How should levothyroxine be dosed for those who are pregnant and have hypothyroidism?
Increase thyroid medication by 30-50%
What are the monitoring parameters for those with hypothyroidism and are pregnant? List them according to the 3 trimesters and the value of each target.
Target TSH
1st trimester < 2.5 mIU/l
2nd trimester < 3.0 mIU/l
3rd trimester < 3.5 mIU/l
Define subclinical hypothyroidism
Elevated TSH with normal T4
What are the risks associated with subclinical hypothyroidism
TSH > 7.0 mIU/L : Increased risk of HF
TSH > 10 mIU/L: Increased risk of CHD
When should subclinical hypothyroidism be treated?
TSH > 10 mIU/L
TSH 4.5 - 10 mIU/L with:
1. Symptoms of hypothyroidism
2. TPO antibodies present
3. History of CVD, heart failure and other risk factors
What is the starting dose for levothyroxine for subclinical hypothyroidism?
25 - 75 mcg
Define hyperthyroidism
Overabundance of circulating thyroid hormone that mimic effects of an activates sympathetic nervous system
What are the primary and secondary causes of hyperthyroidism?
Primary causes: Toxic diffuse goiter (TRAb mimicking TSH); Pituitary adenoma; Toxic adenoma; Toxic multinodular goiter
Secondary causes: Drug induced; subacute thyroiditis due to infections or early Hashimoto’s disease
What are the signs and symptoms associated with hyperthyroidism?
Weight loss and increased appetite
Heat intolerance
Goiter
Fine hair
Tachycardia
Nervous, anxiety, insomnia
Menstrual disturbances (lighter and less frequent)
Sweating
Exophthalamus: bulging eyes
How should I diagnose hyperthyroidism?
Check for signs and symptoms
Do thyroid function test: High free T4 and low TSH
Consider radioactive iodine uptake to check for amount of iodine uptaked by thyroid gland
What are the pharmacological therapies to consider for hyperthyroidism?
Surgical resection
Radioactive iodine ablative therapy (DO NOT use in pregnancy)
Thyroidectomy
Antithyroid pharmacotherapy
List the three agents for antithyroid pharmacotherapy
Thioamides
Iodides
Nonselective beta blockers
What is the indication for use of oral antithyroid pharmacotherapy?
Those awaiting for ablative therapy and surgical resection
Not ablative / surgical candidates that failed to normalize thyroid hormones
Mild disease
Small goiter
Low or negative antibody titer
Women - too young/ old; pregnant; cannot go through radiotherapy due to disability to isolate themselves
What is the mode of action of thinoamides?
What is the special mode of action associated with PTU?
Inhibits iodination and synthesis of thyroid hormones
PTU can block T4 to T3 conversion at periphery at high doses
What are the adverse effects of thionamides?
Hepatotoxicity
Rashes linked to SJS
Agranulocytosis early in therapy
Fever
Cite the monitoring parameters for those on thoionamides and justify the monitoring parameter
FT4 should be monitored every 6-8 weeks
TSH remains suppressed for months even after therapy begins
Can thioamides be 100% effective?
No. Not solving root cause of issue
What are some signs and symptoms associated with those having hyperthyroidism and pregnancy?
Failure to gain weight despite good appetite
Tachycardia
What agent is considered for 1st trimester hyperthyroidism and pregnancy? Why is the other agent not preferred?
PTU
Carbimazole can cause increase risk of congenital malfunctions
What agent is considered for 2nd and 3rd trimester hyperthyroidism and pregnancy? Why is the other agent not preferred?
Carbimazole
PTU can increase risk of hepatotoxicity
What is the mechanism of action for non-selective beta blockers (e.g. propanolol)?
Block many hyperthyroidism manifestations by beta-adrenergic receptors
Block T4 to T3 conversion at high doses
What are some indications for use of nonselective beta blocker?
High risk patients such as elderly with CVD
Treat thyroiditis
Symptomatic relief
Bridging therapy for thionamides effect to kick in / before ablation / surgery
What is the mode of action of iodides?
Inhibit release of stored TH
Decrease vascularity and size of thyroid gland
Decrease effect of hormone synthesis
Define the place in therapy for iodides and why is this so
Before surgery (7-10 days) : shrink thyroid gland
After surgery (3-7 days): Inhibit thyroiditis-mediated release of stored hormones
Thyroid storm
What are some special considerations associated with the use of iodides?
Limit its use to 7-14 days as it thyroid hormone secretion continues after that
Define subclinical hyperthyroidism
Low/undetectable TSH with normal T4
What are some risks associated for subclinical hyperthyroidism? Who are more likely to get these risk?
Age > 60 years more likely to get Afib
Postmenopausal women more likely to get bone fracture
What are some compelling indications for one to initiate therapy for subclinical hyperthyroidism
Young age
TSH < 0.10 mIU/L
What is the agent for subclinical hypothyroidism with AFib?
Beta blockers - propanolol
List the drugs responsible for inducing thyroid disease
Lithium and Amiodarone
How does lithium induce thyroid disorders?
Hypothyroidism: inhibition of thyroid hormone secretion and release cause increased TSH and goiter
Hyperthyroidism: Thyroiditis
How does amiodarone induce thyroid disorders?
Contains iodine structure that can affect how iodine is uptake, secreted and produced
Thyroiditis also can cause hyper/ hypothyroidism
