Pharmacotherapy of Hypothyroidism

Overview

1. Drug of Choice: Levothyroxine (T4)

• Consistent, predictable potency and chemical stability
• Long plasma half-life (~7 days), enabling once-daily dosing
• Peripheral conversion by deiodinases (Dio1, Dio2) to active T3 closely mimics normal physiology
• Approximately 20% of circulating T3 normally derives directly from the thyroid; the remainder comes from peripheral conversion - levothyroxine monotherapy maintains this via tissue deiodination

2. Pharmacokinetics

3. Dosing

Standard Adult Dosing

• Average full replacement dose: ~1.7 mcg/kg/day (approximately 125 mcg/day for a 70 kg adult)
• Older adults (>65 years): ~1.6 mcg/kg/day; lower lean body mass reduces requirement
• Post-thyroidectomy TSH suppression (thyroid cancer): ~2.2 mcg/kg/day

Age-Specific Dosing

Initiating Therapy

• Younger patients / mild disease: Full replacement dose can be started immediately
• Patients >50 years, no cardiac disease: Start at 50 mcg/day
• Elderly / long-standing hypothyroidism / cardiac disease: Start at 12.5-25 mcg/day, increase by 12.5-25 mcg every 2 weeks until euthyroid or toxicity occurs

In patients with coronary artery disease, low thyroid hormone levels paradoxically protect the heart from increased oxygen demand. Overly rapid correction can provoke angina, arrhythmia, or MI. Coronary revascularization should precede aggressive T4 replacement if indicated.

4. Monitoring

• Children: Monitor for normal growth and development
• Elderly: Monitor for atrial fibrillation and bone mineral density (chronic overtreatment accelerates osteoporosis)

5. Administration Pearls

• Optimal: Take on an empty stomach, 30-60 minutes before breakfast, or at bedtime (4 hours after last meal)
• Avoid co-administration with: calcium supplements, iron salts, proton pump inhibitors, antacids, cholestyramine, soy, bran, coffee - all reduce absorption
• Administer at least 4 hours apart from prenatal vitamins and calcium in pregnant patients

Conditions Requiring Higher Doses

• Malabsorptive states: atrophic gastritis, H. pylori gastritis, celiac disease, lactose intolerance, bariatric surgery
• Pregnancy (see below)

6. Special Clinical Situations

A. Subclinical Hypothyroidism

• TSH >10 mIU/L: Levothyroxine generally recommended, especially in younger patients (<65-70 years)
• TSH <10 mIU/L with symptoms: Consider a trial of levothyroxine; discontinue if symptoms do not improve
• Elderly (>80-85 years) with TSH ≤10 mIU/L: Wait-and-watch strategy recommended (age-related TSH rise may not require treatment)

B. Hypothyroidism in Pregnancy

• Overt hypothyroidism is associated with miscarriage, preterm delivery, fetal distress, and impaired psychoneural development in offspring
• Increase dose by ~25-30% as soon as pregnancy is confirmed (practical tip: two extra tablets per week)
• TSH targets by trimester:
  • 1st trimester: 0.1-2.5 mIU/L
  • 2nd trimester: 0.2-3.0 mIU/L
  • 3rd trimester: 0.3-3.0 mIU/L
• Administer T4 at least 4 hours apart from prenatal vitamins/calcium
• Revert to pre-pregnancy dose the day after delivery; recheck TSH at 6 weeks postpartum

C. Myxedema Coma (Emergency)

• Manage in ICU; may require intubation and mechanical ventilation
• IV Levothyroxine loading dose: 300-400 mcg, then 50-100 mcg IV daily
• Can add IV Liothyronine (T3): 5-20 mcg initially, then 2.5-10 mcg every 8 hours - but T3 is more cardiotoxic and harder to monitor
• IV Hydrocortisone (e.g., 50-100 mg q6-8h) until concomitant adrenal insufficiency is excluded
• Supportive: passive rewarming, ventilation, cautious IV fluids (risk of water intoxication), treat precipitating cause
• Administer all drugs IV (poor GI absorption in myxedema)

D. Congenital Hypothyroidism

• Diagnosis within 2 weeks of life and prompt treatment with levothyroxine results in normal physical and intellectual development
• Delayed diagnosis leads to cretinism (irreversible intellectual disability)
• Doses are weight-based and higher per kg than adults

E. Drug-Induced Hypothyroidism

• If the causative drug can be stopped, thyroid function may recover
• If not, manage with levothyroxine
• Amiodarone: Levothyroxine may be required even after discontinuation, given amiodarone's very long half-life (~40-55 days)

7. Combination T4 + T3 Therapy

• Multiple controlled trials show no consistent superiority of combination therapy
• Limitations acknowledged: trials did not focus on patients who remain symptomatic despite normal TSH/free T4, and no long-acting T3 formulation exists
• Desiccated thyroid extract (DTE) (e.g., Armour Thyroid): contains both T4 and T3 in a ~4:1 ratio; some patients express preference (particularly for weight-related outcomes in crossover studies), but lacks consistent evidence of superiority
• A subgroup with Dio2 Thr92Ala polymorphism (impaired T4-to-T3 conversion) might theoretically benefit - but evidence remains insufficient to guide routine testing

8. Toxicity / Overtreatment

9. Key Drug Interactions with Levothyroxine

Recent Evidence Note

• Katzung's Basic and Clinical Pharmacology, 16th Edition
• Goodman & Gilman's The Pharmacological Basis of Therapeutics
• Tietz Textbook of Laboratory Medicine, 7th Edition

Pharmacotherapy of Hypothyroidism

Therapeutic Preparations Available

1. Levothyroxine - Drug of Choice

Why T4 is Preferred Over T3 or DTE

• Consistent, predictable potency and chemical stability
• Long plasma half-life (~7 days) due to extensive protein binding, enabling once-daily dosing
• Peripheral deiodination (by Dio1 and Dio2 enzymes) converts T4 to active T3 in tissues, maintaining steady free T3 levels - closely mimicking normal physiology
• Multiple controlled trials confirm it is not inferior to combination T4+T3

Note: ~20% of circulating T3 normally derives directly from the thyroid gland; the remaining 80% comes from peripheral T4 deiodination. Levothyroxine monotherapy relies on this pathway and is generally sufficient.

2. Pharmacokinetics of Levothyroxine

3. Dosing

Standard Adult Dosing

• Average full replacement: ~1.7 mcg/kg/day (~125 mcg/day for a 70 kg adult); based on lean body mass
• Older adults (>65 years): ~1.6 mcg/kg/day; lower body mass and slower clearance reduce requirement
• Post-thyroidectomy TSH suppression (thyroid cancer): ~2.2 mcg/kg/day

Age-Specific Dosing

Initiating Therapy - Clinical Approach

Cardiac caveat: In patients with coronary artery disease, low thyroid hormone paradoxically protects the heart by reducing oxygen demand. Overly rapid correction can precipitate angina, arrhythmia, or myocardial infarction. If coronary revascularization is indicated, it should be performed before aggressive T4 replacement. Reduce or stop T4 immediately if angina or arrhythmia develops.

4. Monitoring

5. Administration Pearls

Factors Reducing Levothyroxine Absorption

• Foods: Bran, soy, soy formula (infants), coffee, grapefruit, papaya, dietary fiber
• Drugs: Calcium carbonate, ferrous sulfate (iron), cholestyramine, colestipol, sucralfate, antacids (aluminum, magnesium), PPIs
• Conditions requiring higher doses: Malabsorption syndromes, celiac disease, atrophic gastritis, H. pylori gastritis, lactose intolerance, post-bariatric surgery, small bowel surgery

Separate levothyroxine from any of the above by at least 4 hours.

Factors Increasing Dose Requirement

• Pregnancy
• Estrogen/oral contraceptives (increase TBG)
• Rifampin, phenytoin, carbamazepine (hepatic enzyme induction, faster T4 clearance)
• Sertraline (accelerates T4 clearance)
• Post-thyroidectomy patients (tend to need higher doses than those with autoimmune thyroiditis)

6. Special Clinical Situations

A. Subclinical Hypothyroidism (SCH)

• TSH >10 mIU/L, age <65-70 years: Levothyroxine recommended
• TSH <10 mIU/L with symptoms: Consider a levothyroxine trial; discontinue if symptoms don't improve
• TSH ≤10 mIU/L, age >80-85 years: Watchful waiting - age-related TSH rise may not require treatment

B. Hypothyroidism in Pregnancy

• Increase levothyroxine dose by ~25-30% as soon as pregnancy is confirmed - practical approach: two extra tablets per week
• TSH targets by trimester:
  • 1st trimester: 0.1-2.5 mIU/L
  • 2nd trimester: 0.2-3.0 mIU/L
  • 3rd trimester: 0.3-3.0 mIU/L
• Separate T4 from prenatal vitamins and calcium by at least 4 hours
• Increase TBG from estrogen elevates total T4 - use TSH as the primary guide
• Revert to pre-pregnancy dose the day after delivery; recheck TSH at 6 weeks postpartum

C. Myxedema Coma (Emergency)

Patients have large empty T3/T4 binding-protein pools; these must be saturated before free thyroid hormone can act - hence the large loading dose.

D. Congenital Hypothyroidism

• Neonatal screening (heel-prick TSH) allows early detection
• Treatment within the first 2 weeks of life with levothyroxine allows normal physical and intellectual development
• Delayed treatment leads to cretinism (irreversible cognitive impairment, short stature)
• Initial dose: 10-15 mcg/kg/day orally (crushed tablet mixed with breast milk or water); higher end for severe cases
• Monitor free T4 and TSH every 2 weeks initially, then every 1-3 months in the first year
• Soy formula may impair absorption - dose increase may be needed

E. Drug-Induced Hypothyroidism

• Remove offending agent if possible
• If drug cannot be stopped, treat with levothyroxine
• Amiodarone: T4 replacement may be required even after drug discontinuation due to its extremely long half-life (~40-55 days)

F. Central (Secondary/Tertiary) Hypothyroidism

• TSH unreliable for monitoring - use free T4
• Dose adjusted to maintain free T4 in upper third of reference range
• Critical: Always exclude concomitant central adrenal insufficiency before starting levothyroxine - starting T4 without replacing cortisol can precipitate an adrenal crisis (T4 accelerates cortisol clearance)

G. Thyroid Cancer (TSH Suppression)

• Higher levothyroxine doses used to suppress TSH as a growth factor for thyroid cancer
• Average dose: ~2.2 mcg/kg/day
• TSH targets depend on risk stratification:
  • Low-risk/no persistent disease: low-normal TSH
  • High recurrence risk: ~0.1 mU/L
  • Persistent disease: <0.1 mU/L

7. Combination T4 + T3 Therapy

• No long-acting T3 preparation exists, so T3 causes peaks and troughs
• Existing trials show no consistent superiority over T4 alone
• Current T4/T3 preparations don't match the thyroid's natural ~11:1 T4:T3 secretion ratio
• Generic-to-generic levothyroxine switching does not cause clinically significant TSH fluctuations

• ~20% of circulating T3 normally comes directly from the thyroid; T4 monotherapy doesn't perfectly replicate this
• A subset with Dio2 Thr92Ala polymorphism (impaired intracellular T4→T3 conversion) may benefit - but evidence is currently insufficient for routine testing
• Some patients persistently feel unwell despite normal TSH/free T4 on levothyroxine alone; trials have not studied this group specifically
• A crossover study showed some patients preferred desiccated thyroid extract (DTE) with associated weight loss

8. Toxicity from Overtreatment

9. Recent Evidence (2025-2026)

Deltopectoral Flap

Important clarification: The question says "deltopectoral free flap," but the deltopectoral flap is classically a pedicled axial pattern flap - not a free flap. It was historically used as a pedicled regional flap, and in rare instances the internal mammary vessels have been used as a pedicle for free transfer (a technique described but rarely employed). This response covers the classic pedicled deltopectoral flap comprehensively, as this is the clinically standard usage.

1. Historical Background

• Pectoralis major myocutaneous flap (Ariyan, 1979) - single-stage, muscle bulk
• Free tissue transfer (radial forearm, anterolateral thigh, jejunal flaps)

2. Flap Classification

3. Vascular Anatomy

4. Flap Boundaries and Dimensions

5. Anomalous Pivot Point

• There is considerable laxity of skin on the anterior axillary fold when the arm is abducted
• This means the lower border of the flap is longer than the upper border
• Therefore, the effective pivot point is at the medial end of the upper limb of the flap - not at the lower limb
• This must be factored into flap planning and transfer; failure to appreciate this leads to miscalculation of reach

6. Flap Elevation - Surgical Technique

1. Marking: Landmarks are identified - clavicle superiorly, acromion laterally, anterior axillary fold inferiorly, sternal border medially
2. Elevation begins laterally, lifting the flap from lateral to medial
3. The pectoral fascia is included with the flap, leaving the underlying pectoralis major muscle fibres bare
4. Any branches of the acromiothoracic axis encountered must be ligated (these are not the flap's blood supply and their inclusion risks flap elevation complications)
5. Monopolar diathermy is used judiciously - excessive use can damage the flap or leave marks on exposed muscle that compromise subsequent skin graft take
6. Retraction during elevation is upward using skin hooks - the flap must not be folded back on itself, which risks buttonholing
7. The donor site is covered with a split-skin graft

7. Clinical Uses

• Rotated directly to resurface anterior cervical skin defects

• Passed over normal skin as a bridge to a distant defect
• After 3 weeks (once the flap tip has established a new blood supply from the recipient bed), the pedicle is divided
• The remaining flap segment can be returned to the donor site or discarded

• Pedicle is inserted into part of the defect to facilitate vascular ingrowth
• Pedicle is then divided inferiorly and the flap inset into the remainder of the defect

• Most commonly used today to close a fistula after failed hypopharyngeal reconstruction or post-laryngectomy fistula
• Note: where muscle bulk is also required, myocutaneous flaps are preferred

• Used via a controlled fistula approach for circumferential pharyngeal defects
• Largely replaced by free jejunal flap and anterolateral thigh flap

• Thin, pliable skin well suited to external facial resurfacing

8. Advantages

9. Disadvantages and Complications

10. The Deltopectoral Flap as a Free Flap

• The internal mammary vessels are not easily harvested without significant morbidity (rib cartilage removal required to improve mobility)
• Superior free flap donor sites now exist (radial forearm, ALT, jejunum) with more reliable pedicle anatomy and longer vascular leashes

11. Place in Modern Head and Neck Reconstruction

1. Local cervical flaps (random pattern, high failure) - pre-1960s
2. Deltopectoral flap (first axial flap, 1965) - transformative
3. Pectoralis major myocutaneous flap (1979) - single-stage reliability
4. Free tissue transfer (radial forearm, jejunum, ALT) - current gold standard

Surgical Anatomy of the Mastoid Bone

1. Overview and Development

• May be extensive: involving the mastoid tip, squama, petrous apex, and even adjacent zygoma and occipital bone
• May be limited: to a single antral cell
• Non-pneumatized mastoids contain solid cortical bone or diploic (spongy) bone with fatty marrow

2. Gross Anatomy and Surface Landmarks

3. Mastoid Antrum

4. Tegmen

• Oriented in the horizontal plane, just inferior to the arcuate eminence (which marks the top of the superior semicircular canal)
• A low-lying tegmen is not uncommon - the floor of the middle fossa can deepen laterally to form a groove above the attic and labyrinth
• Low-hanging dura may cover the roof of the external auditory canal - a surgical hazard in congenital atresia cases
• Fractures or erosion here allow spread of mastoid infection to the middle cranial fossa (subdural empyema, meningitis)

5. Sigmoid Sinus

• Forms a shallow S-shaped groove on the posterior aspect of the mastoid
• Varies considerably in position: in some individuals it curves anteriorly and sits very close to the posterior EAC wall (especially in patients with chronic ear disease - a hazard during mastoidectomy)
• Only a thin bony plate (the bony plate of the sigmoid sinus or sinus plate) separates it from the mastoid cells
• Trautmann's triangle: bounded by the sigmoid sinus posteriorly, the labyrinth anteriorly, and the superior petrosal sinus superiorly - this is the surgical corridor for some posterior fossa approaches

6. Mastoid Segment of the Facial Nerve (CN VII)

• The mastoid (vertical) segment is the longest segment of the intratemporal facial nerve (10-14 mm)
• Runs vertically downward in the posterior wall of the tympanic cavity and anterior wall of the mastoid
• Extends from the second genu (just distal to the pyramidal process) to the stylomastoid foramen at the skull base
• Direction: travels posteromedially to anterolaterally from second genu to stylomastoid foramen

Surgical Landmarks for the Facial Nerve

Key rule: The facial nerve is almost always lateral to the level of the tympanic annulus, but may cross it or lie medial to it in its lower half - so lowering the facial ridge in canal-wall-down procedures risks injury.

7. Semicircular Canals

• Its dome projects into the floor of the antrum and is a key intraoperative landmark
• Identifies the level of the second genu of the facial nerve (nerve lies just inferomedial to dome)
• The posterior semicircular canal (PSCC) lies just posterior and inferior to the LSCC - at risk during posterior tympanotomy and extended mastoidectomy

8. Jugular Bulb and Jugular Fossa

• Highly variable in size - may be small or may extend superiorly to the petrous ridge
• A high-riding jugular bulb may:
  • Extend into the hypotympanum or even mesotympanum
  • Be dehiscent (without bony cover) within the middle ear - may contact the tympanic membrane
  • Cause pulsatile tinnitus
  • Bleed profusely if inadvertently entered during tympanostomy tube placement or mastoid surgery
  • Mimic a glomus jugulare on otoscopy or MRI

9. Carotid Artery

• Normally covered by the floor of the tympanic cavity
• Can be aberrant/displaced laterally into the middle ear - enters via enlarged inferior tympanic canaliculus, courses through entire middle ear, and reconnects to the horizontal ICA
• An aberrant ICA can be mistaken for a middle ear mass on otoscopy - catastrophic if biopsied or entered surgically

10. Körner's Septum

• Must be recognized and removed to gain access to the true mastoid antrum
• Failure to identify it can lead to false dissection planes

11. Key Spaces and Relationships Summary

12. Surgical Application - Canal Wall-Up Mastoidectomy Steps

1. Identify temporal line (superior limit) and posterior EAC (anterior limit)
2. Remove cortical bone at temporal line - begin exenterating mastoid air cells
3. Skeletonize tegmen plate - trace medially toward antrum
4. Identify and skeletonize sigmoid sinus posteriorly
5. Remove all cells between tegmen and sigmoid sinus
6. Identify and open the mastoid antrum
7. Identify dome of LSCC at antrum floor
8. Identify short process of incus in fossa incudis
9. Skeletonize mastoid segment of facial nerve
10. Open facial recess (triangle bounded by facial nerve, chorda tympani, and incus buttress) - provides access to middle ear without disturbing EAC wall

The three most critical intraoperative principles: (1) always trace the tegmen medially to find the antrum; (2) identify the sigmoid sinus to define the posterior limit; (3) use the LSCC dome and short process of incus to triangulate the facial nerve before drilling near it.

Pharmacotherapy of Hypothyroidism

Available Preparations

1. Levothyroxine - Drug of Choice

• Consistent, predictable potency and chemical stability
• Long plasma half-life (~7 days) → once-daily dosing
• Peripheral deiodination (Dio1, Dio2) converts T4 → active T3 in tissues, mimicking normal physiology
• ~80% of circulating T3 derives from peripheral T4 conversion; levothyroxine relies on this pathway
• Multiple controlled trials confirm no inferiority compared to combination T4+T3

2. Pharmacokinetics

3. Dosing

Standard Adult Doses

Age-Specific Dosing

Initiating Therapy

Cardiac caveat: In coronary artery disease, low thyroid hormone levels protect the heart from increased oxygen demand. Overly rapid correction can provoke angina, arrhythmia, or MI. If coronary revascularization is needed, perform it before aggressive T4 replacement.

4. Monitoring

5. Administration Pearls

Substances Reducing Absorption (separate by ≥4 hours)

• Calcium carbonate, iron salts
• Cholestyramine, colestipol
• Proton pump inhibitors, antacids (Al/Mg)
• Sucralfate
• Soy, bran, dietary fiber, coffee

Conditions Requiring Higher Doses

• Malabsorption: celiac disease, atrophic gastritis, H. pylori gastritis, lactose intolerance
• Post-bariatric surgery / small bowel resection
• Pregnancy
• Enzyme-inducing drugs (rifampin, phenytoin, carbamazepine)

6. Special Clinical Situations

A. Subclinical Hypothyroidism (SCH)

B. Hypothyroidism in Pregnancy

• Increase levothyroxine by ~25–30% as soon as pregnancy is confirmed (practical: two extra tablets per week)
• TSH targets: 1st trimester 0.1–2.5 mIU/L; 2nd trimester 0.2–3.0 mIU/L; 3rd trimester 0.3–3.0 mIU/L
• Separate T4 from prenatal vitamins/calcium by ≥4 hours
• Revert to pre-pregnancy dose the day after delivery; recheck TSH at 6 weeks postpartum

C. Myxedema Coma (Emergency)

D. Congenital Hypothyroidism

• Treatment within first 2 weeks of life → normal physical and intellectual development
• Delayed diagnosis → cretinism (irreversible cognitive impairment, short stature)
• Initial dose: 10–15 mcg/kg/day orally (crushed tablet in breast milk/water)
• Monitor free T4 and TSH every 2 weeks initially, then every 1–3 months in the first year
• Soy formula impairs absorption - dose increase may be needed

E. Drug-Induced Hypothyroidism

• Remove offending agent if possible; otherwise treat with levothyroxine
• Amiodarone: Replacement may be required even after stopping, due to its ~40–55 day half-life

F. Central (Secondary/Tertiary) Hypothyroidism

• Monitor with free T4, not TSH (pituitary TSH secretion is abnormal)
• Goal: maintain free T4 in upper third of reference range
• Always exclude coexisting central adrenal insufficiency before starting levothyroxine - starting T4 without cortisol replacement can precipitate an adrenal crisis

G. Thyroid Cancer (TSH Suppression)

• Low-risk / no persistent disease: low-normal TSH
• High recurrence risk: ~0.1 mU/L
• Persistent disease: <0.1 mU/L

7. Combination T4 + T3 Therapy

• No long-acting T3 formulation exists (causes peaks and troughs)
• Existing trials show no consistent superiority over T4 alone
• No T4/T3 preparation matches the thyroid's natural ~11:1 ratio

• Patients persistently symptomatic despite clearly normal TSH/free T4 on levothyroxine
• Possible Dio2 Thr92Ala polymorphism carriers (impaired intracellular T4→T3 conversion) - though routine testing is not yet evidence-based
• Desiccated thyroid extract (DTE): Some crossover studies show patient preference and weight loss benefit, but evidence for clinical superiority is insufficient

8. Drug Interactions with Levothyroxine

9. Toxicity from Overtreatment

10. Recent Evidence (2025–2026 Updates)

• A 2026 network meta-analysis (PMID 39290156) on treatment preferences found evidence for some benefit of non-levothyroxine-alone regimens in patient-reported outcomes
• A 2026 meta-analysis (PMID 40579157) found that liothyronine-containing regimens are associated with reduced risk of dementia and mortality compared to levothyroxine alone - a finding that may prompt future guideline revision, though current guidelines still recommend T4 monotherapy as first-line