Thyroid Pharmacology · Module 3 of 4

Hyperthyroidism: Thionamides and Storm Management

Drug selection rules, adverse effects, and thyroid storm protocol

Neither thionamide releases preformed thyroid hormone — both block only new synthesis, so clinical improvement requires 2–4 weeks as the existing 2–3 month colloid hormone store depletes; beta-blockade must be started simultaneously to control adrenergic symptoms (tachycardia, tremor, anxiety) during this lag, and should be tapered gradually — not stopped abruptly — as thionamide control is established.
Four rules govern thionamide selection: (1) methimazole for all non-pregnant adults and children; (2) PTU in the first trimester because methimazole embryopathy (aplasia cutis, choanal atresia) occurs in weeks 6–10; (3) switch from PTU to methimazole at 16 weeks to avoid prolonged exposure to PTU's black-box fulminant hepatotoxicity risk; (4) PTU in thyroid storm because its type 1 deiodinase inhibition lowers T3 ~40% faster than methimazole can.
Agranulocytosis is a class effect of both thionamides — 0.1–0.5% incidence, typically within 90 days, presenting as abrupt fever and sore throat; routine blood count monitoring does not prevent it because the reaction is idiosyncratic; every patient must be instructed explicitly to stop the drug immediately and go to an emergency department, not wait for an appointment; do not rechallenge with the other thionamide.
Iodide sequencing is non-negotiable — thionamide must precede iodide by at least 1 hour in every clinical situation (thyroid storm, pre-operative preparation, iodine-excess thyrotoxicosis); giving iodide first provides substrate for new thyroid hormone synthesis before the Wolff-Chaikoff organification block is established, potentially worsening thyrotoxicosis acutely.
Avoid salicylates in thyroid storm — aspirin and other salicylates displace T4 and T3 from plasma binding proteins (TBG, transthyretin, albumin), acutely raising free hormone at the moment end-organ stress is maximal; use acetaminophen exclusively for fever and pain; the storm protocol is PTU load → propranolol → hydrocortisone → iodide (≥1 h after PTU) → adjuncts (cholestyramine, acetaminophen, IV fluids).

Abbreviations: PTU = propylthiouracil · TPO = thyroid peroxidase · T3 = triiodothyronine · T4 = thyroxine · D1 = type 1 deiodinase · TSH = thyroid-stimulating hormone · TSI = thyroid-stimulating immunoglobulin · ANCA = antineutrophil cytoplasmic antibody · SSKI = saturated solution of potassium iodide · TBG = thyroid-binding globulin · G-CSF = granulocyte colony-stimulating factor · ATA = American Thyroid Association

Methimazole vs. Propylthiouracil

Preferred Agent — All Non-Pregnant Adults

Methimazole

Bioavailability ~93%; intrathyroidal concentration allows once-daily dosing
10× more potent than PTU on a milligram basis
Blocks TPO (organification + coupling) only — no peripheral conversion effect
Hepatotoxicity: cholestatic pattern, mild, generally reversible
Teratogenic weeks 6–10: aplasia cutis, choanal atresia, esophageal atresia (embryopathy)
Use: all non-pregnant adults; switch back to methimazole at 16 weeks of pregnancy

Special Situations Only

Propylthiouracil (PTU)

Bioavailability 50–75%, variable; ~80% protein-bound; t½ 1–2 h → three times daily dosing
Blocks TPO AND type 1 deiodinase → lowers T3 ~40% faster than methimazole
Lower placental transfer per milligram → 1st trimester preference
Hepatotoxicity: fulminant hepatic necrosis (FDA black box) — liver failure, transplant, death
ANCA-positive vasculitis: up to 4% with long-term use
Use: 1st trimester pregnancy; thyroid storm (D1 inhibition critical)

Shared Adverse Effects and Selection Rules

Class Effect — Both Thionamides

Agranulocytosis — Patient Education Non-Negotiable

Incidence 0.1–0.5%; typically within first 90 days of treatment
Immune-mediated destruction of granulocyte precursors
Idiosyncratic — routine blood counts do NOT prevent it
Every patient must be told: stop drug immediately if fever or sore throat occurs; go to emergency department; do not wait for appointment
G-CSF accelerates granulocyte recovery
Do NOT rechallenge with the other thionamide — class effect

Four Clinical Selection Rules

When to Use Each Agent

Rule 1: Methimazole for all non-pregnant adults and children — first-line
Rule 2: PTU in 1st trimester (methimazole embryopathy weeks 6–10)
Rule 3: Switch to methimazole at 16 weeks — avoid prolonged PTU fulminant hepatitis risk
Rule 4: PTU in thyroid storm — D1 inhibition provides faster T3 reduction critical in crisis
Block-and-replace contraindicated in pregnancy — thionamide crosses placenta more than levothyroxine at required doses

Thyroid Storm — Mandatory Treatment Sequence

Step 1 — FIRST

PTU 500–1000 mg loading dose (PO or NG tube), then 200–250 mg q4 h — blocks TPO and D1

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Step 2

Propranolol IV (0.5–1 mg q5 min) or PO (60–80 mg q4–6 h) — adrenergic control + D1 inhibition at high dose

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Step 3

Hydrocortisone 100 mg IV q8 h — secretion inhibition, D1 inhibition, adrenal insufficiency coverage

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Step 4 — ≥1 h after PTU

Lugol's iodine or SSKI — Wolff-Chaikoff block on hormone release; iodide AFTER thionamide, never before

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Step 5 — Adjuncts

Cholestyramine (interrupts enterohepatic recirculation); acetaminophen (NOT salicylates); IV fluids; thiamine; treat precipitant

Iodide Sequencing Rule + Salicylate Warning

Iodide must follow thionamide by at least 1 hour — in thyroid storm, pre-operative preparation, and any situation combining both drugs. Giving iodide first provides substrate that can be incorporated into new thyroid hormone synthesis before the Wolff-Chaikoff organification block is established, potentially worsening thyrotoxicosis acutely at a physiologically critical moment.

Avoid salicylates in thyroid storm. Aspirin and other salicylates displace T4 and T3 from plasma binding proteins (TBG, transthyretin, albumin), acutely raising free hormone concentrations when end-organ stress is already maximal. Fever and pain management in thyroid storm: acetaminophen only.

Adjunctive agents and their mechanisms: beta-blockers (propranolol preferred) relieve adrenergic symptoms within hours and inhibit D1 at high doses; glucocorticoids inhibit thyroid hormone secretion, reduce T4→T3 conversion via D1 inhibition, and cover adrenal insufficiency; cholestyramine interrupts enterohepatic recirculation of thyroid hormones and can reduce circulating levels within days in refractory cases.

Suggested References

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th ed. — Chapter 40: Estrogens, Progestins, and the Female Reproductive Tract	McGraw-Hill; 2021
Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 44: Estrogens and Progestins	McGraw-Hill; 2023
Ross DS et al.	2016 ATA guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis	Thyroid. 2016;26(10):1343–1421
Kahaly GJ et al.	2018 European Thyroid Association guideline for the management of Graves’ hyperthyroidism	Eur Thyroid J. 2018;7(4):167–186
Cooper DS.	Antithyroid drugs	N Engl J Med. 2005;352(9):905–917
Bahn Chair RS et al.	Hyperthyroidism and other causes of thyrotoxicosis: ATA and AACE management guidelines	Thyroid. 2011;21(6):593–646
Akamizu T.	Thyroid storm: a Japanese perspective	Thyroid. 2018;28(1):32–40
Alexander EK et al.	2017 ATA guidelines for thyroid disease during pregnancy and postpartum	Thyroid. 2017;27(3):315–389
Haugen BR et al.	2015 ATA management guidelines for thyroid nodules and differentiated thyroid cancer	Thyroid. 2016;26(1):1–133