Drug Monograph
Levothyroxine (T4)
levothyroxine sodium — Synthroid, Levoxyl, Tirosint, Euthyrox, Unithroid
Pharmacokinetic Profile
Half-Life
6–7 days (euthyroid)
Bioavailability
40–80% (oral; fasting)
Protein Binding
>99% (TBG, TBPA, albumin)
Tmax
~3 hours (oral)
Volume of Distribution
~11.6 L (euthyroid)
Metabolism
Deiodination (T4 → T3 + rT3); liver primary
Clinical Information
Drug Class
Synthetic thyroid hormone (L-thyroxine)
Available Doses
25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg tablets
Route
Oral and IV
Renal Adjustment
No specific adjustment; monitor TSH
Hepatic Adjustment
No specific adjustment; monitor TSH
Pregnancy
Essential; increase dose ~30–50%
Lactation
Compatible; normalises milk production
Therapeutic Index
Narrow
Generic Available
Yes (multiple manufacturers)
Indications for Levothyroxine
| Indication | Approved Population | Therapy Type | Status |
|---|---|---|---|
| Hypothyroidism (primary, secondary, tertiary — congenital or acquired) | Adults and pediatric patients, including neonates | Replacement or supplemental therapy | FDA Approved |
| Pituitary TSH suppression in well-differentiated thyroid cancer | Adults | Adjunct to surgery and radioiodine | FDA Approved |
Levothyroxine is the gold-standard treatment for hypothyroidism from any cause, as recommended by both the ATA (2014) and AACE/ATA (2012) guidelines. It is a synthetic L-thyroxine (T4) identical to the hormone produced by the thyroid gland. In the body, approximately 80% of T4 is converted to the active hormone triiodothyronine (T3) by peripheral deiodinases. Because levothyroxine has a narrow therapeutic index, careful dose titration guided by TSH monitoring is essential to avoid the consequences of under- or overtreatment.
Off-Label Uses
Subclinical hypothyroidism (TSH 4.5–10 mIU/L with normal FT4): Treatment is considered when TSH >10 mIU/L, in the presence of symptoms, positive TPO antibodies, or in women planning pregnancy. Routine treatment for mild subclinical hypothyroidism remains controversial. (Evidence quality: Moderate)
Euthyroid goitre suppression: Not recommended in iodine-sufficient populations due to risk of iatrogenic hyperthyroidism and lack of clear benefit (ATA). (Evidence quality: Low)
Myxedema coma: IV levothyroxine is FDA-approved for this indication (200–500 mcg initial IV loading dose). Oral levothyroxine should not be used to treat myxedema coma. (Evidence quality: Moderate)
Levothyroxine Dosing
Adult Dosing by Clinical Scenario
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Primary hypothyroidism — healthy adults <50 years, no cardiac disease | 1.6 mcg/kg/day (full replacement) | Titrate by 12.5–25 mcg q4–6 weeks to normalise TSH | Individualised; rarely exceeds 200–300 mcg/day | Can start at full replacement dose in younger, otherwise healthy patients Steady-state takes 4–6 weeks per dose change due to ~7-day half-life |
| Primary hypothyroidism — elderly (≥65 years) or known cardiac disease | 12.5–25 mcg/day | Increase by 12.5–25 mcg q4–6 weeks | Individualised to TSH target | Lower starting doses reduce risk of angina, arrhythmia, and myocardial infarction Atrial fibrillation is the most common arrhythmia with overreplacement (FDA PI) |
| Severe or longstanding hypothyroidism (non-myxedema) | 12.5–25 mcg/day | Increase by 25 mcg q2–4 weeks | Full replacement (~1.6 mcg/kg/day) | Gradual titration essential to avoid precipitating cardiac events Monitor both clinical symptoms and TSH; do not chase rapid TSH normalisation |
| Hypothyroidism in pregnancy — pre-existing | Increase pre-pregnancy dose by ~30% upon confirmation of pregnancy | Titrate to trimester-specific TSH targets (ATA 2017) | Individualised; may need 1.8 mcg/kg/day or higher | Check TSH q4 weeks in first half, then at least once at ~30 weeks Return to pre-pregnancy dose immediately after delivery; recheck TSH at 6–8 weeks postpartum |
| Hypothyroidism in pregnancy — newly diagnosed | 1.8 mcg/kg/day | Titrate q4 weeks to TSH target | Individualised | Untreated hypothyroidism increases risk of miscarriage, preeclampsia, and impaired fetal neurodevelopment |
| TSH suppression — differentiated thyroid cancer (high-risk) | >2 mcg/kg/day | Titrate to TSH <0.1 mIU/L (high-risk) or 0.1–0.5 mIU/L (intermediate-risk) | As required by TSH target | Balance oncological benefit against cardiovascular and bone risks of iatrogenic thyrotoxicosis ATA 2015 thyroid cancer guidelines define risk-stratified TSH targets |
| Secondary / tertiary hypothyroidism (pituitary or hypothalamic) | 1.6 mcg/kg/day (or as clinical status permits) | Titrate to free T4 in upper half of normal range | Individualised | TSH is unreliable for monitoring; use free T4 as primary target Rule out and treat adrenal insufficiency before starting levothyroxine |
Pediatric Dosing
| Clinical Scenario | Starting Dose | Maintenance Dose | Maximum Dose | Notes |
|---|---|---|---|---|
| Congenital hypothyroidism — neonates (0–3 months) | 10–15 mcg/kg/day | Maintain FT4 in upper half of normal; TSH <5 mIU/L | Individualised | Initiate within 2 weeks of birth to prevent irreversible neurodevelopmental damage Crush tablet and suspend in 5–10 mL water; give immediately; do not use soy formula |
| Congenital hypothyroidism — infants 3–6 months | 8–10 mcg/kg/day | Adjust to TSH and FT4 | Individualised | Dose requirements decrease progressively with age |
| Congenital hypothyroidism — infants 6–12 months | 6–8 mcg/kg/day | Adjust to TSH and FT4 | Individualised | Continue monitoring FT4 in upper half of normal range |
| Children 1–5 years | 5–6 mcg/kg/day | Adjust to TSH | Individualised | Monitor growth and development at each visit |
| Children 6–12 years | 4–5 mcg/kg/day | Adjust to TSH | Individualised | Overtreatment risks craniosynostosis and accelerated bone age |
| Adolescents >12 years | 2–3 mcg/kg/day | Adjust to TSH; approach adult dosing | ~1.6–1.7 mcg/kg/day | Growth complete: transition to adult maintenance dosing |
Clinical Pearl: Steady State and Dose Adjustments
Due to levothyroxine’s long half-life (~7 days), steady-state serum concentrations are not reached until approximately 4–6 weeks after a dose change. Recheck TSH no sooner than 4–6 weeks after any dose adjustment. Making changes more frequently leads to a “chasing” pattern of over- and undercorrection. In elderly or cardiac patients, increments should be small (12.5–25 mcg) and intervals extended. When switching brands, re-measure TSH in 4–6 weeks as bioequivalence between formulations is not guaranteed.
Pharmacology of Levothyroxine
Mechanism of Action
Levothyroxine sodium is a synthetic preparation of L-thyroxine (T4), the principal hormone secreted by the thyroid gland. T4 itself is a prohormone; approximately 80% of circulating triiodothyronine (T3), the biologically active form, is derived from peripheral monodeiodination of T4 by selenium-dependent type I and type II deiodinase enzymes in the liver, kidney, and other tissues. T3 enters the cell nucleus and binds to thyroid hormone receptors (TR-alpha and TR-beta) that are attached to thyroid hormone response elements on DNA. This hormone-receptor complex activates gene transcription and protein synthesis, producing the wide-ranging metabolic effects of thyroid hormone: increased basal metabolic rate, enhanced gluconeogenesis and glycogenolysis, stimulation of protein synthesis, promotion of lipid mobilisation, and regulation of cardiac output, heart rate, and bone turnover. The hypothalamic-pituitary-thyroid (HPT) axis provides negative feedback: rising T4 and T3 levels suppress hypothalamic TRH and pituitary TSH secretion, while falling levels stimulate them.
ADME Profile
| Parameter | Value | Clinical Implication |
|---|---|---|
| Absorption | 40–80% oral; primarily jejunum and upper ileum. Tmax ~3 h. Increased by fasting; decreased by food, fibre, calcium, iron, PPI, soy, and malabsorption syndromes (FDA PI). | Must be taken on an empty stomach, 30–60 minutes before breakfast. Separating from interacting medications by ≥4 hours is critical to ensure consistent absorption. |
| Distribution | Vd ~11.6 L (euthyroid); ~14.7 L (hypothyroid). >99% protein bound (TBG ~80%, TBPA, albumin). Only ~0.02–0.03% circulates free. | Changes in binding proteins (pregnancy, oestrogen, liver disease, nephrosis) alter total T4 but not free T4. Free T4 is the clinically relevant measurement. |
| Metabolism | Sequential deiodination: ~80% of T4 is deiodinated to T3 (active) and rT3 (inactive) in equal amounts. Liver is the major site. Also undergoes glucuronidation, sulfation, and enterohepatic recirculation. | Drugs that induce hepatic enzymes (phenytoin, carbamazepine, rifampicin) increase T4 clearance and may necessitate dose increases. Amiodarone and beta-blockers inhibit T4-to-T3 conversion. |
| Elimination | t½ 6–7 days (euthyroid); 9–10 days (hypothyroid); 3–4 days (hyperthyroid). ~80% renal excretion; ~20% faecal. Urinary excretion decreases with age. | Long half-life supports once-daily dosing. Steady state achieved in ~4–6 weeks. The full therapeutic effect of a dose change may not be apparent until this time. |
Side Effects of Levothyroxine
Adverse effects of levothyroxine are almost exclusively due to overreplacement (iatrogenic thyrotoxicosis) or, rarely, excipient allergy. When the dose is correct and TSH is within the target range, levothyroxine is generally well tolerated with minimal true adverse effects.
≥10%Very Common (Overreplacement Effects)
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Palpitations / tachycardia | ≥10% when TSH suppressed | Dose-related; most common early complaint of overreplacement; resolve with dose reduction |
| Heat intolerance / excessive sweating | ≥10% when TSH suppressed | Classic thyrotoxicosis symptom; signals need for dose reassessment |
| Tremor / nervousness / insomnia | ≥10% when TSH suppressed | May mimic anxiety disorder; always check TSH before adding psychotropic medication |
| Weight loss / increased appetite | ≥10% when TSH suppressed | Expected metabolic effect of excess thyroid hormone; reverses with dose correction |
1–10%Common
| Adverse Effect | Incidence | Clinical Note |
|---|---|---|
| Headache | 1–10% | May occur early in therapy or with dose changes; usually transient |
| Diarrhoea | 1–10% | Reflects increased GI motility from thyroid hormone excess; dose-dependent |
| Menstrual irregularities | 1–5% | Both under- and overreplacement affect menstrual cyclicity; resolve with TSH normalisation |
| Transient hair loss | 1–5% (early therapy) | Telogen effluvium in the first few months of therapy; usually self-limiting within 2–3 months |
| Muscle weakness / cramps | 1–5% | More common with overreplacement; may be confused with myopathy of hypothyroidism itself |
| Excipient allergy (rash, urticaria) | <1% | Related to dyes (tartrazine), lactose, or acacia in certain formulations; switch to dye-free or gel capsule (Tirosint) |
SeriousSerious (Regardless of Frequency)
| Adverse Effect | Estimated Frequency | Typical Onset | Required Action |
|---|---|---|---|
| Atrial fibrillation | Increased risk with TSH <0.1 mIU/L; ~2–3x risk in elderly | Weeks to months of overreplacement | Reduce dose immediately; cardiology referral; anticoagulation assessment per CHA2DS2-VASc |
| Angina pectoris / myocardial infarction | Rare; higher risk in patients with pre-existing coronary artery disease | Days to weeks after initiation or dose increase | Withhold levothyroxine for 1 week; restart at lower dose after cardiac stabilisation; slow titration |
| Accelerated bone loss / osteoporosis | Increased risk with chronic overreplacement, especially postmenopausal women | Months to years of suppressed TSH | Maintain TSH within reference range unless oncological suppression indicated; DEXA scan; ensure calcium and vitamin D adequacy |
| Seizures (rare, upon initiation) | Very rare | Days to weeks after therapy initiation | Reported primarily in paediatric patients; reduce dose and evaluate |
| Acute adrenal crisis (in patients with undiagnosed adrenal insufficiency) | Rare but life-threatening | Days after initiating levothyroxine | Treat adrenal insufficiency with glucocorticoids BEFORE starting levothyroxine; contraindicated in uncorrected AI |
| Craniosynostosis (paediatric overtreatment) | Rare | Months of excess dosing in infants/children | Maintain age-appropriate TSH and FT4; monitor fontanelle closure and bone age |
DiscontinuationDiscontinuation
Lifelong Therapy
Rarely discontinued
Context: Most hypothyroidism is permanent (autoimmune, post-thyroidectomy, post-radioiodine). Discontinuation leads to return of hypothyroid state within weeks. Never stop abruptly.
Trial Discontinuation
Selected patients only
Context: May be considered after resolution of subacute thyroiditis or postpartum thyroiditis. Taper and recheck TSH in 6–8 weeks. Monitor for recurrence.
Overreplacement: The Primary Safety Concern
Virtually all adverse reactions to levothyroxine stem from overreplacement producing iatrogenic thyrotoxicosis. Older patients are at greatest risk for atrial fibrillation, accelerated osteoporosis, and cardiac ischaemia. For most patients with primary hypothyroidism, the TSH target is 0.5–4.0 mIU/L. In patients receiving TSH-suppressive therapy for thyroid cancer, the risk-benefit of suppression must be individually reassessed at each follow-up.
Drug Interactions with Levothyroxine
Levothyroxine has one of the most extensive drug interaction profiles of any commonly prescribed medication. Interactions can affect absorption from the GI tract, serum protein binding, hepatic metabolism, and peripheral conversion of T4 to T3. Because of its narrow therapeutic index, even modest changes in bioavailability can produce clinically significant under- or overreplacement.
MajorCalcium & Iron Supplements
MechanismChelation in the GI tract reduces levothyroxine absorption
EffectElevated TSH due to reduced levothyroxine efficacy
ManagementSeparate by at least 4 hours; take levothyroxine first, supplements later
FDA PIMajorWarfarin / Oral Anticoagulants
MechanismLevothyroxine increases catabolism of vitamin K-dependent clotting factors
EffectEnhanced anticoagulant effect; increased bleeding risk when hypothyroid patient becomes euthyroid
ManagementMonitor INR closely when initiating or adjusting levothyroxine; warfarin dose reduction often needed
FDA PIModerateProton Pump Inhibitors (PPIs)
MechanismIncreased gastric pH reduces levothyroxine solubility and absorption
EffectDecreased levothyroxine absorption (bioavailability may fall by ~40%); rising TSH
ManagementMonitor TSH when initiating/stopping PPI; dose increase may be needed; consider liquid formulation (Tirosint-SOL), which has no labelled PPI interaction
FDA PIModeratePhenytoin / Carbamazepine / Rifampicin
MechanismHepatic CYP enzyme induction increases T4 degradation; phenytoin also displaces T4 from binding proteins
EffectDecreased total and free T4 (by 20–40%); may need levothyroxine dose increase
ManagementMonitor thyroid function closely when initiating, adjusting, or stopping these agents
FDA PIModerateOestrogen / Oral Contraceptives
MechanismOestrogen increases TBG synthesis, reducing free T4 concentration
EffectRising TSH; may need levothyroxine dose increase (typically 20–40%)
ManagementRecheck TSH 4–6 weeks after starting or stopping oestrogen; adjust dose accordingly
FDA PIModerateAmiodarone
MechanismInhibits type I and type II deiodinase (blocks T4 → T3 conversion); iodine load from amiodarone can cause hypo- or hyperthyroidism
EffectComplex: may cause hypothyroidism (amiodarone-induced) or thyrotoxicosis; T4 increases, T3 decreases
ManagementBaseline and periodic thyroid function tests (q6 months on amiodarone); adjust levothyroxine as needed
FDA PIModerateAntidiabetic Agents (insulin, metformin)
MechanismThyroid hormone replacement increases metabolic rate and glucose turnover
EffectMay worsen glycaemic control; antidiabetic dose may need to be increased
ManagementMonitor glucose closely when initiating or adjusting levothyroxine in diabetic patients
FDA PIMinorBiotin (Vitamin B7)
MechanismBiotin interferes with streptavidin-biotin immunoassays used to measure TSH and T4
EffectFalsely low TSH and falsely high FT4, mimicking thyrotoxicosis on laboratory testing
ManagementStop biotin supplements at least 2 days before thyroid function testing (FDA PI 2024 update)
FDA PIMonitoring for Levothyroxine
- TSH4–6 weeks after initiation or each dose change; then q6–12 months when stable
RoutinePrimary monitoring parameter for primary hypothyroidism. Draw TSH in the morning before levothyroxine dose (trough). Target 0.5–4.0 mIU/L for most adults; lower targets for TSH suppression in thyroid cancer. Unreliable in secondary/tertiary hypothyroidism. - Free T4 (FT4)With TSH at each assessment; primary target in central hypothyroidism
RoutineTarget upper half of normal range. Essential for monitoring in secondary/tertiary hypothyroidism where TSH is unreliable. Useful to assess absorption and compliance. - TSH in PregnancyQ4 weeks in first half; at least once near 30 weeks
RoutineUse trimester-specific reference ranges. Target TSH <2.5 mIU/L in first trimester (or per lab-specific reference if available). Recheck 6–8 weeks postpartum after returning to pre-pregnancy dose. - Cardiac StatusBaseline ECG in elderly/cardiac patients; symptoms at every visit
Trigger-basedMonitor for palpitations, chest pain, and dyspnoea, particularly during titration. Atrial fibrillation is the most common arrhythmia with overreplacement. - Bone Density (DEXA)Baseline and periodic in postmenopausal women on suppressive therapy
Trigger-basedChronic TSH suppression accelerates bone loss. Maintain the minimum dose achieving the desired TSH target. Ensure calcium and vitamin D adequacy. - Growth & Development (Paediatric)Every visit in children
RoutineMeasure height, weight, and developmental milestones. Monitor bone age periodically. In congenital hypothyroidism, maintain FT4 in upper half of normal to support neurodevelopment. - Biotin IntakeInquire before each thyroid function test
Trigger-basedBiotin interferes with immunoassays causing falsely low TSH and falsely high FT4. Stop biotin at least 2 days before blood draw.
Contraindications & Cautions
Absolute Contraindications
- Uncorrected adrenal insufficiency — levothyroxine increases metabolic clearance of cortisol and can precipitate an acute adrenal crisis; treat with glucocorticoids first
- Thyrotoxicosis (untreated) — levothyroxine will worsen hyperthyroidism
- Known hypersensitivity to levothyroxine sodium or any excipient in the formulation
Relative Contraindications (Specialist Input Recommended)
- Acute myocardial infarction — withhold or use extreme caution; initiation of thyroid hormone may increase myocardial oxygen demand
- Uncontrolled cardiac arrhythmias — thyroid hormone can exacerbate; start at very low doses under close monitoring
Use with Caution
- Elderly and cardiovascular disease — start 12.5–25 mcg/day; slow titration to avoid angina, MI, and AF (FDA PI)
- Diabetes mellitus — correction of hypothyroidism may worsen glycaemic control; adjust antidiabetic therapy
- Osteoporosis / postmenopausal women — avoid TSH suppression below the reference range unless oncologically indicated; monitor bone density
- Adrenal insufficiency (corrected) — ensure adequate glucocorticoid replacement before and during levothyroxine treatment
FDA Boxed Warning
Not for Treatment of Obesity or Weight Loss
Thyroid hormones, including levothyroxine, should not be used for the treatment of obesity or for weight loss. In euthyroid patients, doses within the range of daily hormonal requirements are ineffective for weight reduction. Larger doses may produce serious or even life-threatening manifestations of toxicity, particularly when given in combination with sympathomimetic amines such as those used for their anorectic effects.
Patient Counselling
Purpose of Therapy
Levothyroxine replaces the thyroid hormone your body is unable to produce in sufficient quantities. For most patients, this is a lifelong medication. It restores normal energy levels, metabolism, weight regulation, and prevents the serious complications of untreated hypothyroidism.
How to Take
Take levothyroxine once daily on an empty stomach, ideally 30 to 60 minutes before breakfast, with a full glass of water. Consistency is key: take it at the same time each day. Do not crush levothyroxine capsules (Tirosint). Tablets may be crushed and suspended in water for infants who cannot swallow tablets. Do not store the suspension — use it immediately.
Timing and Food Interactions
Tell patientTake on an empty stomach, 30–60 minutes before eating or drinking anything other than water. Calcium supplements, iron supplements, antacids, and multivitamins must be taken at least 4 hours apart from levothyroxine. Coffee, soy products, high-fibre foods, and grapefruit juice can also reduce absorption.
Call prescriberIf starting or stopping any new medication, supplement, or dietary pattern that might interfere with absorption; if thyroid symptoms return despite consistent dosing.
Signs of Over- or Underdosing
Tell patientOverdosing (too much thyroid hormone) causes racing heart, palpitations, excessive sweating, tremor, weight loss, anxiety, and insomnia. Underdosing causes fatigue, weight gain, cold intolerance, constipation, dry skin, and low mood. Both situations require dose adjustment — never change the dose without prescriber guidance.
Call prescriberIf experiencing chest pain, rapid or irregular heartbeat, shortness of breath, persistent unexplained fatigue, or significant weight changes.
Consistency of Product
Tell patientLevothyroxine has a narrow therapeutic index, meaning small dose changes can have big effects. Not all levothyroxine brands are interchangeable. If the pharmacy substitutes a different brand or generic, notify your prescriber so that TSH can be rechecked in 4–6 weeks.
Call prescriberIf the pharmacy changes brands without notification; if symptoms of over- or underdosing develop after a switch.
Pregnancy Planning
Tell patientLevothyroxine is safe and essential during pregnancy. If you become pregnant or are planning pregnancy, contact your prescriber immediately — your dose will likely need to increase by about 30% or more. Untreated hypothyroidism during pregnancy carries risks for both mother and baby, including miscarriage and impaired fetal brain development.
Call prescriberImmediately upon confirmation of pregnancy; dose increase should begin as early as possible.
Missed Doses
Tell patientIf you miss a dose, take it as soon as you remember the same day. If it is almost time for the next dose, skip the missed dose and resume the usual schedule. Do not double the dose. Because levothyroxine has a long half-life (~7 days), a single missed dose will not cause immediate problems, but habitual missed doses will lead to worsening hypothyroidism.
Call prescriberIf frequently missing doses; if symptoms of hypothyroidism return.
Storage
Tell patientStore tablets at room temperature (20–25 °C / 68–77 °F). Protect from light and moisture. Do not store in the bathroom. Keep out of reach of children — accidental ingestion in children can cause serious toxicity.
Call prescriberIf accidental ingestion by a child occurs, seek emergency medical attention immediately.
Sources
Regulatory (PI / SmPC)
- AbbVie Inc. Synthroid (levothyroxine sodium) tablets — Full Prescribing Information. Revised February 2024. DailyMed: SynthroidPrimary US prescribing reference for oral levothyroxine; source for dosing tables, PK parameters, drug interactions, and contraindications.
- Pfizer Inc. Levoxyl (levothyroxine sodium) tablets — Prescribing Information. Pfizer Medical: LevoxylComplementary PI confirming PK data; source for bioavailability of tablet vs solution and metabolism parameters.
- Fresenius Kabi USA. Levothyroxine Sodium for Injection — Prescribing Information. FDA LabelPI for IV levothyroxine for myxedema coma; source for IV dosing, estimated IV-to-oral bioavailability (48–74%), and drug interactions.
Guidelines
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670–1751. https://doi.org/10.1089/thy.2014.0028Landmark ATA guideline establishing levothyroxine monotherapy as the standard of care; provides dosing principles, monitoring recommendations, and evidence review.
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the AACE and ATA. Endocr Pract. 2012;18(6):988–1028. https://doi.org/10.4158/EP12280.GLAACE/ATA consensus guideline on hypothyroidism diagnosis and management in adults; supports weight-based dosing and TSH-guided titration.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315–389. https://doi.org/10.1089/thy.2016.0457ATA guideline for thyroid disease in pregnancy; source for trimester-specific TSH targets and dosing adjustments during pregnancy.
- Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association management guidelines for adult patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2016;26(1):1–133. https://doi.org/10.1089/thy.2015.0020ATA thyroid cancer guideline; defines risk-stratified TSH suppression targets for levothyroxine after thyroidectomy.
Key Clinical Trials
- Stott DJ, Rodondi N, Kearney PM, et al. Thyroid hormone therapy for older adults with subclinical hypothyroidism. N Engl J Med. 2017;376(26):2534–2544. https://doi.org/10.1056/NEJMoa1603825TRUST trial: large RCT showing no benefit of levothyroxine for subclinical hypothyroidism (TSH 4.6–19.9 mIU/L) in adults ≥65 years on hypothyroid symptoms or tiredness.
- Braverman LE, Ingbar SH, Sterling K. Conversion of thyroxine (T4) to triiodothyronine (T3) in athyreotic human subjects. J Clin Invest. 1970;49(5):855–864. https://doi.org/10.1172/JCI106304Foundational study demonstrating that ~80% of circulating T3 is derived from peripheral conversion of T4, supporting T4-only replacement strategy.
Mechanistic / Basic Science
- Bianco AC, Salvatore D, Gereben B, Berry MJ, Larsen PR. Biochemistry, cellular and molecular biology, and physiological roles of the iodothyronine selenodeiodinases. Endocr Rev. 2002;23(1):38–89. https://doi.org/10.1210/edrv.23.1.0455Comprehensive review of deiodinase enzymes responsible for T4-to-T3 conversion; contextualises why T4 monotherapy works for most patients.
- Brent GA. Mechanisms of thyroid hormone action. J Clin Invest. 2012;122(9):3035–3043. https://doi.org/10.1172/JCI60047Review of thyroid hormone receptor signalling, gene transcription, and the wide-ranging metabolic effects of T3 at the cellular level.
Pharmacokinetics / Special Populations
- Benvenga S, Bartolone L, Pappalardo MA, et al. Altered intestinal absorption of L-thyroxine caused by coffee. Thyroid. 2008;18(3):293–301. https://doi.org/10.1089/thy.2007.0222Demonstrates that coffee can reduce levothyroxine absorption, supporting the recommendation to avoid coffee for 30–60 minutes after dosing.
- Centanni M, Gargano L, Canettieri G, et al. Thyroxine in goiter, Helicobacter pylori infection, and chronic gastritis. N Engl J Med. 2006;354(17):1787–1795. https://doi.org/10.1056/NEJMoa043903Establishes that impaired gastric acid secretion (H. pylori, PPIs) reduces levothyroxine absorption, necessitating dose increases.
- Biondi B, Wartofsky L. Treatment with thyroid hormone. Endocr Rev. 2014;35(3):433–512. https://doi.org/10.1210/er.2013-1083Extensive review of levothyroxine therapy including PK parameters, dosing in special populations, drug interactions, and long-term safety considerations.