Study guide:
Definition:
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Simply, decrease in circulating thyroid hormones with an increase in TSH.
Types:
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Primary (caused by conditions related to the thyroid gland)
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Secondary (caused by conditions related to pituitary gland)
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Tertiary (caused by condition related to hypothalamus)
Causes:
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Primary causes: (MOST COMMON):
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Hashimatto (chronic thyroiditis):
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Most common cause in developed country.
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Autoimmune.
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Caused by autoantibodies (antithyroperoxidase and antimicrosomal ‘anti-TPO’ antibodies).
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Iodine deficiency:
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Most common cause in developing country.
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Iatrogenic:
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Drugs (lithium, amiodarone).
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RAI (causes thyroid destruction).
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Thyroid removal surgery.
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Congenital (cretinism)
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Diagnosis:
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Best initial/most useful screening test is measuring serum TSH level.
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If TSH is increased:
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Check free T4 → decreased or normal:
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Decreased → primary hypothyroidism.
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Normal → subclinical hypothyroidism (increase in TSH could still manage the problem and cause thyroid hormone secretion).
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If TSH is decreased:
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Check T4 → decreased → Secondary hypothyroidism
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If TSH is normal but the symptoms strongly suggest hypothyroidism:
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Check T4 → decreased or normal:
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Decreased → secondary hypothyroidism → Do imaging (for patient with sarcidosis, head injury, cancer or under radiotherapy)
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Normal → no further evaluation
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If structural abnormalities suspected → US or radioneuclic scanning.
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Secondary and tertiary causes:
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Tumor.
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Necrosis (e.g: shehann).
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Pituitary or hypothalamic failure.
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Transient hypothyroidism:
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Caused by subacute or lymphatic thyroiditis
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Reverses in 2-8 months.
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Symptoms:
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Most common complaint is weakness
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Lethargy
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Cold intolerance
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Weight gain (decreased basal metabolic rate)
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Period irregularities in females (oligomenorrhea)
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Decreased cognition
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Constipation
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Depression
Signs:
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Most common sign is coarse or dry skin
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Slow speech
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Eyelid edema
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Non-pitting edema
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Puffy face
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Carpel tunnel
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Hoarseness
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Goiter (Rubbery, non-tender)
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Increase in cholesterol level
High TSH (X2 of normal) + normal T4 = Treatment!

Treatment:
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Lifelong levothyroxine.
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Dose 1.6 mcg\kg.
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In young healthy individuals with no heart disease → start the full replacement dose
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In individuals who are 60 year and older OR with heart diseases → start with 12.5-25 mcg → then titer 25-50 mcg every 6 weeks.
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In these groups full dose increases the risk of angina, MI, HF, & arrhythmia!
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Needs 6 weeks for TSH to go back to normal.
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With the treatment, keep TSH in the lower half of the normal reference range (0.4-2.5).
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Do not decrease TSH below 0.4 à osteoporosis and atrial fibrillation risks.
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Increase the dose by 75% in case of pregnancy.
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Treat subclinical hypothyroidism in case of:
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Patient with goiter
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Hypercholestremia
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Symptoms of hypothyroidism
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TSH is more than 20
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In case of secondary hypothyroidism give hydrocortisone with the thyroxin.
Follow-up:
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In tittering or any change in dose, asses the TSH every 6 weeks.
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When TSH is normal follow up every 6 months to 1 year.
References:
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Agabegi S, Agabegi E, Ring A. Step-up to medicine. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins; 2013.
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Aafp.org. Thyroid and Parathyroid Disorders - American Family Physician [Internet]. 2015 [cited 23 December 2015]. Available from: http://www.aafp.org/afp/topicModules/viewTopicModule.htm?topicModuleId=67
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Lipsky M, King M. Blueprints family medicine. Baltimore, MD: Lippincott Williams & Wilkins; 2011.
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Sattar H. Fundamentals of pathology. Chicago: Pathoma.com; 2011.
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Kaplan. Kaplan lecture notes, Medicine, 2014.
Written by: Rawan Al-Tuwaijri
Reviewed by: Jumana AlJohani
Bassam Alghamdi
Format Editor: Roaa Amer
Website publisher: Adel Yasky
Bayan Alzomaili