The thyroid makes thyroxine (T4) and triiodothyronine (T3) from iodine under the control of pituitary TSH. T4 is converted to the more active T3 in tissues. Thyroid hormone raises metabolic rate, heat production, heart rate and contractility, and gut motility; it is needed for growth and brain development. Thyroid hormone feeds back to suppress TSH, so TSH is the main test for adjusting therapy.

Hypothyroidism (Hashimoto thyroiditis, post-thyroidectomy or post-RAI, congenital hypothyroidism, TSH suppression after thyroid cancer), myxedema coma (IV levothyroxine). Hyperthyroidism (Graves disease, toxic nodular goiter), preparation for thyroidectomy, and thyroid storm.

Laboratory reference (adults; ranges vary by laboratory)

Levothyroxine — adverse effects are those of too much hormone (iatrogenic hyperthyroidism): tachycardia, palpitations, atrial fibrillation, angina, tremor, nervousness, insomnia, heat intolerance, sweating, weight loss, diarrhea, menstrual changes. Long-term overtreatment causes bone loss (especially postmenopausal clients) and atrial fibrillation in older adults. Undertreatment leaves hypothyroid symptoms.

Thionamides (methimazole and PTU)

• Agranulocytosis — usually in the first 3 months: fever, sore throat, mouth ulcers
• Hepatotoxicity — PTU: hepatocellular injury, liver failure (boxed); methimazole: cholestatic injury
• Rash, itching, arthralgia (common, often mild)
• ANCA-associated vasculitis (more with PTU)
• Methimazole in the first trimester: birth defects (aplasia cutis, choanal and esophageal atresia)
• Hypothyroidism from overtreatment

Iodide — metallic or brassy taste, burning mouth, sore teeth and gums, increased salivation, rash, GI upset (iodism); staining of teeth; hypersensitivity.

Radioactive iodine — neck tenderness, temporary rise in thyroid hormone (can worsen hyperthyroidism briefly), worsening of Graves eye disease (steroid cover may be given), permanent hypothyroidism in most clients.

Levothyroxine

• Uncorrected adrenal insufficiency — correct with glucocorticoid first (thyroid hormone raises cortisol clearance and can trigger adrenal crisis)
• Caution or not recommended: acute MI and untreated thyrotoxicosis
• Start low in older adults and clients with coronary artery disease (e.g., 12.5–25 mcg daily) and increase slowly — full doses can provoke angina or dysrhythmias
• Absorption is reduced by calcium, iron, antacids (aluminum, magnesium), bile acid sequestrants, sucralfate, phosphate binders, soy, high-fiber meals, coffee; separate by about 4 hours; proton pump inhibitors may reduce absorption
• Increases effect of warfarin (more INR monitoring when starting or changing the dose)
• Increases insulin and oral antidiabetic requirements; reduces digoxin levels
• Enzyme inducers (rifampin, carbamazepine, phenytoin) and estrogen may increase requirements
• Pregnancy: requirements rise early — many clients need about a 20–30% dose increase as soon as pregnancy is confirmed; TSH is kept in a lower, trimester-specific range; levothyroxine is safe in pregnancy and breastfeeding and must not be stopped

Thionamides

• Prior agranulocytosis or liver failure with a thionamide
• Pregnancy: PTU in the first trimester, then usually switch to methimazole; the lowest effective dose is used because both cross the placenta (fetal hypothyroidism and goiter)
• Breastfeeding: methimazole at moderate doses is considered compatible
• Warfarin effect may decrease as the client becomes euthyroid; beta-blocker, digoxin, and theophylline doses may need reduction as metabolism normalizes

Iodide — iodine hypersensitivity; not used alone long term (escape); do not give before thionamide in thyroid storm (provides substrate for new hormone).

Radioactive iodine — contraindicated in pregnancy and breastfeeding; avoid pregnancy for about 6 months after treatment.

Beta blockers — asthma (use cardioselective agents with caution), heart block, decompensated heart failure.

Listed in priority order.

1. Thyroid storm (life-threatening) — high fever, severe tachycardia or atrial fibrillation, agitation, delirium, vomiting. Airway, oxygen, cardiac monitoring; give thionamide first (PTU or methimazole), then iodine at least 1 hour later, beta blocker, corticosteroid (hydrocortisone blocks T4 → T3 conversion and covers adrenal reserve), cooling measures, and acetaminophen — not aspirin (salicylates displace thyroid hormone from binding proteins). Treat the trigger.
2. Myxedema coma — IV levothyroxine, IV hydrocortisone until adrenal insufficiency is excluded, ventilatory support, passive rewarming, cautious fluids for hyponatremia, glucose monitoring; avoid sedatives and opioids.
3. Agranulocytosis check — any fever or sore throat in a client on a thionamide: hold the drug and obtain a CBC with differential at once; report ANC below 1,000/µL (1.0 × 10⁹/L) — at that level the thionamide is stopped.
4. Cardiac monitoring with levothyroxine initiation or dose increases in older adults and heart disease — apical pulse before doses; hold and report heart rate above about 100/min, chest pain, or new irregular rhythm.
5. Labs — TSH (and free T4) 6–8 weeks after any levothyroxine dose change, then every 6–12 months when stable; for thionamides, free T4 and T3 every 4–6 weeks at first (TSH lags), baseline CBC and liver tests, and liver tests for symptoms.
6. Levothyroxine administration — same time daily on an empty stomach; separate from interacting drugs; IV doses are roughly half to three-quarters of oral doses — confirm the order.
7. Iodine solutions — dilute in juice or milk and give through a straw to reduce taste and tooth staining.
8. RAI safety — confirm a negative pregnancy test; reinforce radiation precautions.
9. Assess for over- and undertreatment — weight, heart rate, sleep, bowel pattern, heat or cold intolerance, energy.

Levothyroxine

• Take once daily on an empty stomach with water, 30–60 minutes before breakfast (or at bedtime at least 3–4 hours after the last meal, if the prescriber agrees); take the same way every day.
• Separate calcium, iron, antacids, and fiber supplements by about 4 hours.
• Therapy is usually lifelong; improvement takes several weeks; do not stop or change the dose on your own.
• Do not switch brands or generic makers without telling the prescriber — TSH may need rechecking.
• Do not use thyroid hormone for weight loss — it is dangerous at doses that cause weight loss.
• Report palpitations, chest pain, fast heartbeat, nervousness, heat intolerance, or weight loss (too much), and fatigue, cold intolerance, or constipation (too little).
• Tell the prescriber as soon as pregnancy is confirmed — the dose usually needs to go up.

Methimazole or PTU

• Take as prescribed at regular times; improvement takes weeks; do not stop or adjust alone.
• Stop the drug and call at once for fever, sore throat, or mouth sores — a blood test is needed.
• Report yellow skin or eyes, dark urine, pale stools, right upper abdominal pain, or loss of appetite (liver injury).
• Report pregnancy plans or pregnancy immediately (drug switch).

Iodine and RAI

• Take iodine solution diluted, through a straw; report rash or swelling.
• After RAI, follow the program's precautions for several days: limit close and prolonged contact, especially with pregnant women and young children, sleep alone, do not share utensils, flush twice, wash hands well, and drink fluids. Avoid pregnancy for about 6 months. Watch for later symptoms of hypothyroidism.

Levothyroxine overdose — symptoms can be delayed several days because of conversion and the long half-life: tachycardia, dysrhythmias, hypertension, fever, agitation, seizures (rare). Treatment is supportive: beta blockers (propranolol) for adrenergic symptoms, cooling, benzodiazepines for agitation; activated charcoal soon after ingestion; bile acid sequestrants (cholestyramine) may reduce reabsorption. Most small pediatric ingestions are benign but need advice from poison control.

Thionamide toxicity — agranulocytosis: stop the drug, CBC, broad-spectrum antibiotics for fever, granulocyte colony-stimulating factor in some cases; never restart the same class. Liver failure: stop the drug, liver tests, possible transplant evaluation (PTU).

Iodine excess — iodism (stop the drug); iodine-induced hyperthyroidism (Jod-Basedow) or hypothyroidism in susceptible glands.

There is no single antidote; thyroid storm and myxedema coma are the true emergencies (Section 5).

• TSH is the key test: hypothyroid = high TSH, low free T4; recheck 6–8 weeks after a dose change
• Levothyroxine: empty stomach, 30–60 min before breakfast, separate calcium, iron, and antacids by 4 hours; lifelong
• Start low, go slow in older adults and coronary disease; toxicity = hyperthyroid symptoms (tachycardia, AF, angina)
• Boxed warning: not for weight loss
• Levothyroxine raises warfarin effect; raises insulin needs
• Pregnancy: increase levothyroxine 20–30% early; never stop
• Methimazole preferred; PTU for first trimester and thyroid storm (liver toxicity boxed warning)
• Thionamides: fever or sore throat → hold drug, CBC (agranulocytosis)
• Iodine blocks release; give at least 1 hour after the thionamide in storm; dilute, use a straw
• Thyroid storm: thionamide → iodine → beta blocker → steroid; acetaminophen, not aspirin
• Myxedema coma: IV levothyroxine plus IV hydrocortisone; passive rewarming; avoid sedatives
• RAI: exclude pregnancy; avoid pregnancy about 6 months; later hypothyroidism