LevothyroxinePA study guide

Why they happen and how to manage them

Tachycardia, AF, angina

Excess thyroid hormone raises heart rate and contractility and increases myocardial oxygen demand. Management: Start low in older and cardiac patients; reduce the dose if symptoms appear.

Bone loss

Over-replacement speeds bone resorption. Management: Use the lowest effective dose and avoid suppressed TSH unless intended.

Adrenal crisis

Restoring metabolism raises cortisol demand. Management: Replace glucocorticoids before starting.

Narrow therapeutic index

Small dose or absorption changes shift thyroid status. Management: Consistent timing and brand; recheck TSH 6 to 8 weeks after any change.

Sources: [1][2][3][4]

Common and serious

Sources: [1][2][3][4][5]

Narrow therapeutic index

Narrow therapeutic index. The label states that levothyroxine has a narrow therapeutic index, so doses are titrated against TSH.

Sources: [1]

Blood glucose

Why

Worsened diabetes control

When

When thyroid therapy is started, changed, or stopped

Normal

Fasting 70 to 99 mg/dL.

Sources: [1][2]

Heart rate

Why

Overtreatment, tachycardia, atrial fibrillation

When

During titration, especially in older and cardiac patients

Act on

Reduce or pause the dose for signs of overdose

Normal

60 to 100 beats per minute in adults at rest.

Sources: [1][2][3]

INR (prothrombin time)

Why

Stronger warfarin effect

When

Closely when correcting hypothyroidism or raising the dose in patients on warfarin

Act on

The warfarin dose may need to drop

Normal

INR 0.8 to 1.1 when not on a blood thinner. On warfarin, the usual target is 2.0 to 3.0.

Sources: [1][2]

TSH (thyroid-stimulating hormone)

Why

Dose adequacy in primary hypothyroidism

When

6 to 8 weeks after any dose change; every 6 to 12 months when stable. Stop biotin 2 days before

Act on

Titrate by 12.5 to 25 mcg until TSH normalizes

Normal

0.4 to 4.8 µU/mL.

Sources: [1][2][3]

Adult dose

Hypothyroidism

Full replacement is about 1.6 mcg/kg/day. Titrate by 12.5 to 25 mcg every 4 to 6 weeks to euthyroid. More than 200 mcg/day is seldom needed; poor response above 300 mcg/day suggests poor adherence, malabsorption, or interactions.

Older adults or cardiac disease

Start below 1.6 mcg/kg/day and titrate every 6 to 8 weeks.

Sources: [1]

Renal adjustment

The label gives no renal dose adjustment. Dosing is based on age, weight, cardiac status, and TSH or free T4.

Sources: [1][2]

Hepatic adjustment

The label gives no hepatic dose adjustment. Dosing is based on age, weight, cardiac status, and TSH or free T4.

Sources: [1][2]

Boxed warning

Not for obesity or weight loss. Replacement doses do not reduce weight in people with normal thyroid function, and larger doses can cause serious or life-threatening toxicity, especially with sympathomimetic appetite suppressants.

Sources: [1]

Contraindications

• Uncorrected adrenal insufficiency

Sources: [1]

Key interactions

Calcium, iron, phosphate binders, bile acid sequestrants, Kayexalate

Bind levothyroxine and cut absorption. Separate by at least 4 hours.

PPIs, sucralfate, antacids

Less stomach acid means less absorption. Monitor TSH.

Warfarin

Levothyroxine boosts the anticoagulant response. Monitor INR; the warfarin dose may need to drop.

Insulin and oral diabetes drugs

Glucose control may worsen, raising dose needs.

Digoxin

Digoxin levels and effect can fall as the patient becomes euthyroid.

Ketamine or sympathomimetics

Additive cardiovascular stimulation, with hypertension and tachycardia.

Soy, walnuts, dietary fiber, grapefruit juice

Reduce or delay absorption.

Biotin supplements

Falsely alter TSH and T4 tests. Stop biotin 2 days before testing.

Sources: [1][2][3][4][5][6][7][8]

Antidote or reversal

No antidote. Overdose looks like hyperthyroidism, sometimes with confusion, and can be delayed for days. Reduce or pause the dose and give supportive care.

Sources: [1]

Pregnancy

No increase in birth defects or miscarriage with doses that keep the mother euthyroid. Do not stop during pregnancy; TSH often rises, so monitor and raise the dose as needed. Untreated hypothyroidism harms mother and fetus.

Sources: [1]

Lactation

Present in human milk with no reported infant harm. Adequate treatment may help restore low milk supply in hypothyroid mothers.

Sources: [1]

Pediatric

Weight-based starting doses fall with age, from 10 to 15 mcg/kg/day at 0 to 3 months to 1.6 mcg/kg/day once growth is complete. Titrate every 2 weeks. Check TSH and T4 at 2 and 4 weeks, 2 weeks after any change, then every 3 to 12 months. Undertreatment harms cognition and growth; overtreatment can cause craniosynostosis.

Sources: [1][2][3]

Geriatric

Start below full replacement because cardiovascular disease is common. Atrial fibrillation is the most frequent arrhythmia from overtreatment in older adults.

Sources: [1]