Hypothyroidism

Question 1

Risk Factors and Etiologies

Answer 1

• Age-related
• Females > Males
• Iodine deficiency (most common cause worldwide but not in U.S.)
• Autoimmune thyroiditis (most common cause in U.S.)
• Hashimoto’s thyroiditis or chronic autoimmune thyroiditis
• More common in people with other autoimmune diseases such as T1DM,
Addison’s Disease, Down’s Syndrome, Turner’s Syndrome, celiac disease,
rheumatoid arthritis, lupus

Question 2

Symptoms

Answer 2

Common in older adults: Fatigue, dyspnea, changes in taste/hearing or ataxia
• Less common in older adults: Cold intolerance, constipation, weight gain, paresthesia, muscle cramps
• Confounded by comorbidity: Bradycardia, diastolic hypertension, pallor, dry skin, coarse hair, hoarseness,
voice changes, delayed tendon reflexes, depression and mental status
changes
• Thyroid should always be on differential for cognitive changes

Question 3

Thyroid Hormone Relationships

Answer 3

Thyroid hormone secretion regulated by
hypothalamus (TRH) and pituitary (TSH)
•Thyroid releases MORE T4 (100%) than T3 (20%)
•T4 is converted to active T3 in peripheral
tissues
•T3 is primarily responsible for action
•T3 and T4 presence acts as a negative
feedback loop to “turn off” production
•High plasma binding (T4 > T3) predicts half life and interactions

Question 4

Thyroid normal levels

Answer 4

Normal Levels
TSH: 0.1 – 4 mIU/L
fT4: 0.7 – 1.9 ng/dL

Question 5

Subclinical Hypothyroidism

Answer 5

TSH < 10

Normal fT4

Question 6

Overt Hypothyroidism

Answer 6

TSH > 10 ↓ fT4

Question 7

T3 (Liothyronine)

Answer 7

• Peaks 2-4 hrs
• T ½ < 1 day
• Higher risk of thyrotoxicosis

Question 8

T4 (Levothyroxine)

Answer 8

Peaks 2-4 hrs
• T ½ ~ 7 days
• Requires conversion of T4 to T3
• Missed doses have less clinical impact

Question 9

Combination thyroid products

Answer 9

Desiccated porcine thyroid (T3>T4)

Liotrix (T4:T3 = 4:1)

Question 10

Equivalent dosing

Answer 10

• Liothyronine (T3 ) 25 mcg
• Desiccated porcine thyroid (T3>T4) 1 grain/60 mg
• Liotrix (T4:T3 = 4:1) 1 grain (50 mcg T4 + 12.5 mcg T3)
• Levothyroxine (T4) 100 mcg

Question 11

Drug interactions: ↑ Protein or Binding

↓ free concentration/activity

Answer 11

• Estrogen, selective estrogen receptor
modulator (SERMs), methadone, 5-FU
• Liver disease, HIV

Question 12

Drug interactions: ↓ Protein or Binding

↑ free concentration/activity

Answer 12

• Corticosteroids, androgens, furosemide, salicylates, AEDs (i.e., phenytoin)
• Acute illness

Question 13

Absorption → administration, impaired absorption

Answer 13

• Conditions: Celiac disease, chronic diarrhea, GI bypass surgery
• Interactions: Minerals, bile acid sequestrant, fiber supplements, acid suppression therapy, foods (empty stomach preferred)
Other Considerations:
• Some antiepileptic drug and rifampin may increase clearance
• Amiodarone therapy and selenium deficiency may block peripheral conversion

Question 14

Overt Hypothyroidism treatment

Answer 14

treat to therapeutic TSH

• Levothyroxine 50-100 mcg/day (0.8 mcg/lb)

Question 15

Reduce starting dose in older adults?

Answer 15

• Levothyroxine 25-50 mcg/day

* Titrate 12.5-25 mcg every 4-6 weeks

Question 16

Reassess TSH

Answer 16

4-6 weeks after initiation or dose adjustments

Question 17

Risks of Over-Treatment

Answer 17

• Tachyarrhythmias
• Ischemic heart disease
• Palpitations
• Angina
 Heat Intolerance
• Diarrhea
• Osteoporosis
• Fracture
• Anxiety
• Tremulousness
• Irritability
• Insomnia