Thyroid Pharmacology · Module 2 of 4

Hypothyroidism: Clinical Pharmacology

Diagnosis, dosing, TSH targets, and emergencies

Central hypothyroidism requires a monitoring pivot — in primary hypothyroidism the pituitary amplifies any hormone deficit into a large TSH rise, making TSH the endpoint; in central hypothyroidism the pituitary cannot mount this response, so TSH stays low or normal even at subtherapeutic doses, and using TSH to guide dosing leads to chronic under-replacement; free T4 in the upper half of the reference range, checked 6 weeks after any dose change, is the correct endpoint.
The 6-week rule is universal — levothyroxine's 6–7 day half-life requires 4–5 half-lives to reach steady state; TSH checked before 6 weeks reflects a non-equilibrium state and leads to inappropriate dose adjustments; this applies to initiation, any dose change, any formulation switch (including branded-to-generic), and any change in interacting medications.
TSH targets are context-specific, not universal — standard adult 0.5–2.5 mIU/L; elderly (>65) 1.0–4.0 mIU/L (TRUST trial showed no benefit of treating subclinical hypothyroidism in this age group); pregnancy 1st trimester <2.5 mIU/L, 2nd/3rd <3.0 mIU/L with dose increase 25–30% immediately on positive test; thyroid cancer low-risk 0.5–2.0 mIU/L, high-risk <0.1 mIU/L.
Myxedema coma requires glucocorticoids before or with thyroid hormone — adrenal insufficiency co-occurs in pituitary disease and autoimmune polyglandular syndromes, and thyroid hormone replacement without cortisol support can precipitate cardiovascular collapse; empiric IV hydrocortisone 50–100 mg every 6–8 hours is given first, followed by IV levothyroxine 300–500 mcg loading dose then 50–100 mcg/day; IV liothyronine is used at some centers but carries arrhythmia risk.
Combination T4/T3 therapy is second-line and patient-specific — a subset of patients remains symptomatic on adequate levothyroxine due to the type 2 deiodinase Thr92Ala polymorphism (homozygous in ~16%), which impairs local T4→T3 conversion in brain and pituitary; however, randomized trial evidence is inconsistent, other causes of persistent symptoms must be excluded first (celiac disease, adrenal insufficiency, iron deficiency, depression), and combination therapy is contraindicated in cardiac disease, arrhythmia, and osteoporosis without bone protection.

Abbreviations: TSH = thyroid-stimulating hormone · T4 = thyroxine · T3 = triiodothyronine · TPO = thyroid peroxidase · TBG = thyroid-binding globulin · IV = intravenous · PPI = proton pump inhibitor · D2 = type 2 deiodinase · ATA = American Thyroid Association · TRUST = Thyroid hormone Replacement for Untreated older adults with Subclinical hypothyroidism Trial

Primary vs. Central Hypothyroidism

Feature	Primary Hypothyroidism	Central Hypothyroidism
Cause	Thyroid gland failure — Hashimoto's, radioactive iodine, thyroidectomy, lithium, amiodarone, checkpoint inhibitors	Pituitary or hypothalamic disease — macroadenoma, surgery, radiation, Sheehan's, infiltrative disease
TSH	Elevated — pituitary amplifies any T4 deficit into large TSH rise	Low, normal, or only mildly elevated — pituitary cannot respond normally
Free T4	Low	Low
Monitor with	TSH — sensitive and reliable endpoint	Free T4 only — TSH cannot be used; target upper half of reference range
TSH target	0.5–2.5 mIU/L (standard adult)	Not applicable — using TSH as endpoint leads to systematic under-replacement

TSH Targets by Clinical Context

Standard Adult

0.5–2.5 mIU/L

Full replacement: ~1.6 mcg/kg/day
Recheck TSH no sooner than 6 weeks
Obese: use lean body weight
Partial function: 1.0–1.3 mcg/kg/day

Elderly (>65 years)

1.0–4.0 mIU/L

Start 12.5–25 mcg; uptitrate slowly over months
Lower TSH associated with AF and bone loss in elderly
TRUST trial: no benefit from treating subclinical hypothyroidism (TSH 4.5–10) in age >65 without symptoms

Pregnancy

<2.5 / <3.0 mIU/L

1st trimester: TSH <2.5 mIU/L
2nd/3rd trimester: TSH <3.0 mIU/L
Increase dose 25–30% immediately on positive pregnancy test — do not wait for appointment
Monitor TSH every 4 weeks in 1st trimester; every 4–6 weeks thereafter

Thyroid Cancer

Risk-Stratified

Low-risk (after successful ablation): 0.5–2.0 mIU/L
High-risk (persistent/metastatic disease): TSH <0.1 mIU/L
Suppression target balances cancer control against AF and osteoporosis risk

Myxedema Coma — Emergency Protocol

Recognition and Precipitants

Clinical Features — Mortality 20–50%

Depressed consciousness (somnolence to stupor), hypothermia
Hypoventilation, bradycardia, hyponatremia, hypoglycemia
Typically elderly women with undiagnosed or undertreated hypothyroidism
Precipitants: infection (most common — pneumonia, UTI), cold, opioids, sedatives, anesthetics, levothyroxine non-adherence
GI absorption unreliable — IV thyroid hormone required

Pharmacological Treatment

IV Protocol — Glucocorticoid First

IV hydrocortisone 50–100 mg q6–8 h empirically — give before or with thyroid hormone; adrenal insufficiency co-occurs and replacing thyroid hormone without cortisol can precipitate cardiovascular collapse
IV levothyroxine: 300–500 mcg loading dose (saturates expanded volume of distribution); then 50–100 mcg/day IV; reduce dose in elderly or ischemic heart disease
+/− IV liothyronine: used at some centers for impaired conversion; carries arrhythmia risk — no RCT mortality benefit
Supportive: warming, mechanical ventilation, glucose; treat precipitating illness

The 6-Week Rule and Formulation Considerations

TSH must not be rechecked sooner than 6 weeks after any levothyroxine initiation, dose change, formulation switch, or change in a drug that interacts with levothyroxine absorption or metabolism. The 6–7 day half-life requires 4–5 half-lives to reach steady state; earlier measurements reflect non-equilibrium concentrations and drive inappropriate adjustments.

Formulation matters: standard tablets require acidic gastric environment and are impaired by PPIs, achlorhydria, celiac disease, and gastric bypass (proximal small bowel bypassed — often 30–50% dose increase needed). Liquid levothyroxine and soft gelatin capsules are minimally affected by gastric pH or food. Any brand-to-generic switch or formulation change warrants a 6-week TSH recheck — bioavailability can differ by up to 12.5% between formulations.

Combination T4/T3 therapy: second-line only, after excluding other causes of persistent symptoms (celiac disease, adrenal insufficiency, iron deficiency, depression, sleep apnea). If trialed, add liothyronine 5–10 mcg once or twice daily with a corresponding levothyroxine dose reduction; reassess at 3–6 months. Contraindicated in cardiac disease, significant arrhythmia, frailty, and osteoporosis without bone protection.

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
Jonklaas J et al.	Guidelines for the treatment of hypothyroidism: American Thyroid Association task force	Thyroid. 2014;24(12):1670–1751
Garber JR et al.	Clinical practice guidelines for hypothyroidism in adults: AACE and ATA	Thyroid. 2012;22(12):1200–1235
Stott DJ et al.	Thyroid hormone therapy for older adults with subclinical hypothyroidism (TRUST trial)	N Engl J Med. 2017;376(26):2534–2544
Biondi B et al.	Subclinical hypothyroidism: a review	JAMA. 2019;322(2):153–160
Alexander EK et al.	2017 ATA guidelines for diagnosis and management of thyroid disease during pregnancy	Thyroid. 2017;27(3):315–389
Kwaku MP, Burman KD.	Myxedema coma	J Intensive Care Med. 2007;22(4):224–231
Bianco AC, Kim BW.	Deiodinases: implications of the local control of thyroid hormone action	J Clin Invest. 2006;116(10):2571–2579
Ross DS et al.	2016 ATA guidelines for diagnosis and management of hyperthyroidism	Thyroid. 2016;26(10):1343–1421