CH 61 Drugs for Thyroid Disorders

Question 1

Drug: Levothyroxine (Synthroid)

Answer 1

Class: Synthetic thyroid hormone (T4)
MOA: Replaces endogenous thyroxine (T4), which is converted to triiodothyronine (T3); regulates metabolism, growth, and development.
Dosing:
* Adults: 1.6mcg/kg/day; increase by 25mcg/day every 4-6weeks until TSH in normal range
* 60+ years: 1.1mcg/kg/day (2/3 of that needed in younger adult)

Note:
* Different brands are not considered interchangeable due to variable bioavailability

Question 2

Drug: Armour Thyroid

Answer 2

Class: Natural thyroid hormone (T3/T4 combination)
MOA: Provides both T3 and T4 derived from porcine thyroid glands; mimics endogenous thyroid hormone effects.
Note:
* Has a strong, characteristic odor due to its animal origin.
* Prescribed in grains

Question 3

Drug: Methimazole (MMI)

Answer 3

Class: Thioamide antithyroid agent
MOA: Inhibits thyroid peroxidase, preventing synthesis of new thyroid hormones by blocking iodination of tyrosine residues.
AE:
* agranulocytosis
* teratogenicity

Note: First-line therapy for hyperthyroidism

Question 4

Drug: Propylthiouracil (PTU)

Answer 4

Class: Thioamide antithyroid agent
MOA: Inhibits thyroid hormone synthesis and also blocks peripheral conversion of T4 to T3.
AE:
* liver injury
* agranulocytosis

Note: Preferred during the first trimester of pregnancy due to lower teratogenic risk

Question 5

Drug: Radioactive Iodine (¹³¹I)

Answer 5

Class: Radiopharmaceutical
MOA: Selectively taken up by the thyroid and destroys thyroid tissue via beta-emission.
Note: Contraindicated in pregnancy (Category X); does not emit systemic radiation.

Question 6

Drug: Propranolol

Answer 6

Class: Non-selective beta-adrenergic blocker
MOA: Reduces sympathetic symptoms of hyperthyroidism and may inhibit peripheral conversion of T4 to T3.
Note: Used in thyroid storm for both symptom control and hormone suppression.

Question 7

Drug: Iodides (e.g., Lugol’s solution, potassium iodide)

Answer 7

Class: Iodine preparation
MOA: Inhibits the release of preformed thyroid hormone from the gland and decreases vascularity of the thyroid.
Note: Given after a thioamide in thyroid storm to prevent release of stored hormone.

Question 8

Function of thyroid hormone

Answer 8

• Regulates metabolism
• Affects growth & development
• Controls energy, body temp, heart rate
• Essential for brain development

Question 9

Hypothalamic-pituitary-thyroid feedback loop

Answer 9

• Hypothalamus → TRH → Anterior Pituitary → TSH
• TSH stimulates thyroid to release T3 & T4
• High T3 & T4 inhibit TRH and TSH release

Question 10

Signs of hyperthyroidism

Answer 10

• Weight loss
• heat intolerance
• Increased heart rate
• tremors
• Anxiety
• bulging eyes (exophthalmos)
• Goiter

Question 11

Signs of hypothyroidism

Answer 11

• Weight gain
• cold intolerance
• Fatigue
• constipation
• Dry skin
• slow heart rate
• Coarse skin
• slow reflexes

Question 12

Triiodothyronine (T3)

Answer 12

• the active thyroid hormone
• more potent thyroid hormone replacement therapy
• shorter half-life

Synthetic (T3): Liothyronine

Question 13

Thyroxine (T4)

Answer 13

• the precursor to triiodothyronine (T3)
• more common; preferred thyroid hormone replacement therapy
• stable half-life

Synthetic (T4): Levothyroxine

Question 14

overt primary HYPERthyroidism

Answer 14

TSH: ↓↓ | Total T4: ↑↑ | Free T4: ↑↑ | Total T3: ↑↑

Causes: Graves’ Disease, toxic multinodular goiter, toxic adenoma, thyroditis (virual, postpartum), drugs, hyperemesis gravidarum, gestational throphoblastic disease, excess thryoxine ingestion

Question 15

sublclinical primary HYPERthyroidism

Answer 15

TSH: ↓ or ↓↓ | Total T4: NL | Free T4: NL | Total T3: NL

Causes: Recent treatement for hyperthyroidism, steroids, dopamine, non-thyroid illness

Question 16

overt primary HYPOthyroidism

Answer 16

TSH: ↑↑ | Total T4: ↓↓ | Free T4: ↓↓ | Total T3: ↓↓

Causes: Hashimoto’s thyroiditis, thyroidectomy, post-radioiodine therapy, amyloidosis, neck irradiation

Question 17

subclinical primary HYPOthyroidism

Answer 17

TSH: ↑ or ↑↑ | Total T4: NL | Free T4: NL | Total T3: NL

Causes: Poor compliance with thyroxine, drugs (amiodarone), assay difficulties; non-thyroid illness (including acute psychiatric disorders)

Question 18

central (secondary) HYPERthyroidism

Answer 18

TSH: ↑↑ | Total T4: ↑↑ | Free T4: ↑↑ | Total T3: ↑↑

Causes: TSH-secreting pititary adenoma, hypothalamic dysfunction

Question 19

central (secondary) HYPOthyroidism

Answer 19

TSH: ↓↓ | Total T4: ↓↓ | Free T4: ↓↓ | Total T3: ↓↓

Causes: Pituitary dysfunction, non-thyroid illness, assay difficulties, hypothalamic dysfunction

Question 20

Drugs that decrease levothyroxine absorption

Answer 20

• histamine blockers
• proton pump inhibitors (PPI)
• sucralfate
• cholestyramine
• antacids (aluminum-containing)
• iron supplements
• magnesium salts

Question 21

Drugs that increase levothyroxine metabolism

Answer 21

These drugs would require an increased levothyroxine dose
* phenytoin
* carbamazepine
* rifampin
* sertraline
* phenobarbital

Question 22

Drugs needing monitoring with levothyroxine

Answer 22

• warfarin - ↑INR; ↓ warfarin dose
• insulin - ↑ insulin dose
• digoxin
• corticosteroids

Question 23

Changes in thyroid dosing during pregnancy

Answer 23

• thyroid binding globulin (TBG) increases throughout pregnancy
• ↑TBG = ↓ free T4
• Increase levothyroxin (Synthroid) by 50% during pregnancy
• Return to pre-pregnancy levels following delivery

Question 24

Thyroid Storm treatment

Answer 24

1. propylthiouracil (PTU) OR methimazole (Tapazole) - direct antithyroid (prevent synthesis of new T3/T4)
2. Lugol’s solution OR sodium iodide - prevent stored T3/T4 from being secreted
3. propranolol - will decrease thyroid hormone production
4. hydrocortisone - will decrese inflammatory processes

Avoid aspirin