Thyroid Pharmacology · Module 1 of 4

Thyroid Hormone Physiology and Pharmacokinetics

Axis, biosynthesis, deiodinases, receptors, and drug interactions

The TSH–free T4 relationship is log-linear — a twofold change in free T4 produces approximately a 100-fold change in TSH, making TSH the most sensitive single marker of thyroid hormone status; this amplification also means a TSH of 8 mIU/L represents a far smaller hormone deficit than a TSH of 80 mIU/L, and it explains why levothyroxine steady state (6–7 day half-life, reached in ~6 weeks) must be achieved before TSH reflects the true dose effect.
Thyroid hormone biosynthesis proceeds in sequence through NIS-mediated iodide trapping → thyroid peroxidase organification → TPO-mediated coupling → secretion, and each step is a pharmacological target: NIS is blocked by perchlorate and exploited by radioactive iodine; TPO is blocked by methimazole and PTU; the Wolff-Chaikoff effect (iodide excess transiently inhibiting organification) is exploited by Lugol's solution preoperatively and by iodide given after thionamides in thyroid storm.
The three deiodinase isoforms govern peripheral thyroid hormone activation and inactivation — type 1 (liver, kidney, muscle) converts T4 to T3 and is inhibited by PTU, amiodarone, and high-dose propranolol; type 2 (pituitary, brain) locally generates intracellular T3 for TSH feedback and is upregulated in hypothyroidism; type 3 (placenta, fetal tissues) inactivates T4 and T3, protecting the developing fetus — it is also upregulated in critical illness, producing the low-T3/high-rT3/normal TSH pattern of sick euthyroid syndrome.
TR-alpha1 (heart, bone, gut) and TR-beta1/2 (liver, pituitary) have distinct tissue distributions explaining organ-specific effects — tachycardia and atrial fibrillation reflect TR-alpha1 excess; elevated LDL in hypothyroidism reflects TR-beta1 deficiency at the liver; resistance to thyroid hormone syndrome involves TR-beta2 mutations, producing elevated T4/T3 with paradoxically non-suppressed TSH because the pituitary cannot sense the hormone, while peripheral TR-alpha1 tissues remain sensitive and develop tachycardia.
Amiodarone disrupts thyroid physiology through at least four concurrent mechanisms: massive iodine load (37% iodine by weight) triggering Wolff-Chaikoff effect; type 1 deiodinase inhibition (raises rT3, lowers T3); direct cytotoxicity on follicular cells; and inhibition of T4 cellular entry — the net result can be either hypothyroidism (most common) or two mechanistically distinct thyrotoxicosis syndromes requiring different treatments, mandating thyroid function testing before starting and every 6 months during therapy.

Abbreviations: TRH = thyrotropin-releasing hormone · TSH = thyroid-stimulating hormone · T4 = thyroxine · T3 = triiodothyronine · rT3 = reverse triiodothyronine · NIS = sodium-iodide symporter · TPO = thyroid peroxidase · MIT = monoiodotyrosine · DIT = diiodotyrosine · PTU = propylthiouracil · TR = thyroid hormone receptor · TBG = thyroid-binding globulin · hCG = human chorionic gonadotropin · LDL = low-density lipoprotein · PPI = proton pump inhibitor

Hypothalamic-Pituitary-Thyroid Axis

Hypothalamus

TRH released into portal circulation → stimulates pituitary thyrotrophs; dopamine and somatostatin tonically inhibit TSH

→

Pituitary

TSH secreted (alpha subunit shared with LH, FSH, hCG); log-linear inverse relationship with free T4 — 2× T4 change = ~100× TSH change

→

Thyroid

TSH drives iodide trapping, hormone synthesis, colloid resorption, and secretion — ~80–100 µg T4/day; only 5–10 µg T3/day

→

Feedback

T3 (local, from type 2 D2) suppresses TSH at pituitary; ~80% of circulating T3 from peripheral T4 conversion, not direct secretion

Biosynthesis Steps and Drug Targets

Step 1 — NIS

Iodide Trapping

Concentrates iodide 20–40× plasma; TSH-upregulated
Exploited by radioactive iodine (I-131): enters via NIS → beta radiation destroys follicular cells
Perchlorate: competitive NIS inhibitor; used in iodine-excess thyrotoxicosis
High iodide: downregulates NIS expression (Wolff-Chaikoff escape mechanism)

Steps 2–3 — TPO

Organification and Coupling

TPO + H₂O₂ (DUOX2) → iodide oxidized → attached to thyroglobulin tyrosyl residues → MIT and DIT
Coupling: DIT + DIT → T4; DIT + MIT → T3
Methimazole: blocks organification and coupling — preferred agent
PTU: blocks organification and coupling + inhibits type 1 deiodinase (unique advantage in thyroid storm)

Wolff-Chaikoff Effect

Iodide Excess → Transient Block

Acute iodide loading transiently inhibits TPO-mediated organification
Lugol's iodine (7–10 days preoperatively): reduces gland vascularity before thyroidectomy
Thyroid storm: iodide given ≥1 hour after thionamide to block ongoing hormone release
Escape occurs after days via NIS downregulation — iodide is not durable antithyroid therapy

Deiodinase Isoforms — Tissue Distribution and Drug Effects

Type 1 (D1) — Peripheral

T4 → T3 Conversion; rT3 Clearance

Liver, kidney, thyroid, skeletal muscle
Primary source of circulating T3 (~80% from peripheral conversion)
Inhibited by PTU, amiodarone, and high-dose propranolol → raises rT3, lowers T3
PTU advantage over methimazole: D1 inhibition blocks ongoing T3 production in thyroid storm

Type 2 (D2) — CNS / Pituitary

Local T4 → T3 for TSH Feedback

Pituitary, brain, heart
Generates intracellular T3 for TSH negative feedback — makes TSH exquisitely sensitive to circulating T4
Upregulated in hypothyroidism (local buffer); downregulated when T4 rises
Not significantly inhibited by PTU or amiodarone

Type 3 (D3) — Placenta / Fetus

Inactivation: T4 and T3 → rT3

Placenta, fetal tissues, skin
Protects fetus from premature thyroid hormone exposure during neurodevelopment
Upregulated in critical illness (sick euthyroid syndrome) → low T3, elevated rT3, normal-low TSH
Sick euthyroid: do not treat — levothyroxine in critical illness has not been shown to reduce mortality; recheck 4–6 weeks post-recovery

Drug Interactions Affecting Levothyroxine

Interaction Type	Drugs	Mechanism	Management
Absorption reducers	Calcium, iron, antacids (Al/Mg), cholestyramine, sucralfate, PPIs	Bind levothyroxine in GI tract; PPIs raise gastric pH → reduced tablet dissolution	Separate by ≥4 hours; take levothyroxine on empty stomach 30–60 min before breakfast
Metabolism inducers	Rifampin, phenytoin, carbamazepine, phenobarbital	CYP enzyme and glucuronidation induction → accelerated T4 hepatic clearance	Dose increase 20–50% typically required; recheck TSH after starting or stopping
TBG elevation	Oral estrogen, pregnancy	Increases TBG production → raises total T4; free T4 and TSH remain unchanged in euthyroid individuals	No dose change needed if TSH normal; monitor in hypothyroid patients — may need dose increase
hCG cross-reactivity	First-trimester pregnancy, gestational trophoblastic disease	hCG shares alpha subunit with TSH; high hCG cross-reacts with TSH receptor → suppresses TSH, stimulates thyroid output	Gestational hyperthyroidism: usually self-limited; thionamides if severe and symptomatic

Amiodarone — Multiple Mechanisms, Two Distinct Thyrotoxicosis Syndromes

Amiodarone disrupts thyroid hormone pharmacology through four concurrent mechanisms: (1) massive iodine load (37% iodine by weight) triggering Wolff-Chaikoff organification block; (2) type 1 deiodinase inhibition — raises rT3, lowers T3; (3) direct inhibition of T4 cellular entry; (4) direct cytotoxic effect on thyroid follicular cells. Check thyroid function before starting and every 6 months during therapy.

Amiodarone-induced hypothyroidism (most common): TSH rises; treat with levothyroxine; amiodarone can usually be continued.

Amiodarone-induced thyrotoxicosis type 1: excess iodine drives autonomous synthesis in a pre-existing nodular or Graves' thyroid; treat with thionamides (methimazole or PTU) ± potassium perchlorate to block NIS. Amiodarone-induced thyrotoxicosis type 2: destructive thyroiditis from direct follicular cell toxicity → thyroid hormone leaks; treat with glucocorticoids (prednisone). The two types frequently coexist — combined thionamide plus glucocorticoid therapy is used when the type is uncertain.

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
Ortiga-Carvalho TM et al.	Hypothalamus-pituitary-thyroid axis	Compr Physiol. 2016;6(3):1387–1428
Portulano C et al.	The Na+/I− symporter (NIS): mechanism and medical impact	Endocr Rev. 2014;35(1):106–149
Peeters RP, Visser TJ.	Metabolism of thyroid hormone	Endotext. MDText.com, 2017
Bianco AC et al.	Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases	Endocr Rev. 2002;23(1):38–89
Cheng SY et al.	Molecular aspects of thyroid hormone actions	Endocr Rev. 2010;31(2):139–170
Jonklaas J et al.	Guidelines for the treatment of hypothyroidism: American Thyroid Association task force	Thyroid. 2014;24(12):1670–1751
Ross DS et al.	2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism	Thyroid. 2016;26(10):1343–1421