Thyroid Medications

Thyroid Hormone Synthesis and Metabolism

• The thyroid gland synthesizes thyroxine (T4/L-thyroxine) and triiodothyronine (T3/L-triiodothyronine) through iodination of tyrosine residues in thyroglobulin, a process requiring thyroid peroxidase (TPO) enzyme and iodine as essential cofactor. T4 comprises approximately 80% of thyroid hormone secretion and serves as a prohormone, while T3 (constituting ~20% of secretion) is the biologically active form with 3-4 times greater receptor affinity than T4. Peripheral conversion of T4 to T3 occurs primarily in liver (60%), kidney (20%), and other tissues through deiodinase enzyme activity; this conversion is regulated by multiple factors including TSH, selenium status, beta-adrenergic tone, and tissue-specific metabolic demand. The hypothalamic-pituitary-thyroid (HPT) axis maintains homeostasis through thyrotropin-releasing hormone (TRH) secretion from the hypothalamus, which stimulates thyroid-stimulating hormone (TSH) from the anterior pituitary; TSH binds to TSH receptor on thyroid follicular cells, activating adenylyl cyclase and increasing hormone synthesis and secretion. Free thyroid hormones (free T4 and free T3) constitute only ~0.3% and ~0.1% of total hormone respectively, with the remainder bound to thyroid-binding globulin (TBG), transthyretin, and albumin; these free fractions represent the biologically available hormone and feedback-regulate the HPT axis.

Mechanism of Levothyroxine Action

• Levothyroxine (synthetic L-T4) is absorbed in the proximal small intestine through active transporter-mediated mechanisms (organic anion transporter 1B1 [OATP1B1] and others), with absorption influenced by gastric pH, intestinal transit time, concurrent medications, and food intake. Once absorbed, levothyroxine distributes broadly throughout body tissues with a large volume of distribution (approximately 50 L), and approximately 99.9% becomes protein-bound in circulation. The drug undergoes peripheral monodeiodination to T3 (active form) and reverse T3 (inactive metabolite) through type 1, 2, and 3 deiodinases; this conversion is the primary mechanism by which exogenous T4 provides biological activity, making levothyroxine a prodrug that requires tissue-specific enzymatic activation. T3 crosses the cell membrane through specific transporters (monocarboxylate transporter 8 [MCT8] being the primary transporter) and binds to thyroid hormone receptors (TRα and TRβ) in the cytoplasm and nucleus, forming heterodimereric complexes with retinoid X receptors (RXRs) that bind to thyroid hormone response elements (TREs) in promoter regions of target genes. This transcriptional regulation increases expression of genes encoding metabolic enzymes (mitochondrial glycerol-3-phosphate dehydrogenase), contractile proteins, and metabolic regulators; at physiological concentrations, thyroid hormones increase basal metabolic rate by 20-40%, enhance heat production, increase oxygen consumption, and stimulate protein synthesis. The elimination of levothyroxine occurs primarily through hepatic deiodination (~35%), with subsequent urinary excretion of iodine; the biological half-life of approximately 7 days permits once-daily dosing and explains the 4-6 week timeline required to reach steady state after dose adjustments.

Mechanism of Antithyroid Drugs: PTU and Methimazole

• Propylthiouracil (PTU) and methimazole are small-molecular-weight thiouracil derivatives that inhibit thyroid hormone synthesis through multiple complementary mechanisms. Both drugs inhibit thyroid peroxidase (TPO), the enzyme essential for iodine organification and coupling of iodotyrosyl residues to form T3 and T4 within thyroglobulin; this action occurs at the apical membrane of thyroid follicular cells and represents their primary mechanism. PTU possesses a unique additional mechanism: inhibition of type 1 deiodinase (D1) in hepatic and other peripheral tissues, reducing the conversion of circulating T4 to T3 and thus decreasing active hormone concentrations; methimazole lacks this peripheral deiodinase inhibition and therefore has minimal effect on T3 conversion. Both drugs are absorbed rapidly from the gastrointestinal tract, distribute minimally across the blood-brain barrier (except PTU, which crosses somewhat more readily), and are metabolized hepatically. PTU undergoes hepatic metabolism via glucuronidation and sulfation with a half-life of 1-2 hours, necessitating divided dosing; methimazole is metabolized to active metabolites and has a longer half-life of 4-6 hours, permitting once-daily dosing. Importantly, antithyroid drugs do NOT directly affect circulating thyroid hormones already present—they prevent only new hormone synthesis—so clinical improvement requires time for circulating hormones to be metabolized and cleared (typically 1-2 weeks for PTU, 3-5 weeks for methimazole, reflecting half-life differences).

Beta-Adrenergic Mechanisms in Thyrotoxicosis

• Thyroid hormones enhance beta-adrenergic receptor sensitivity through upregulation of beta-adrenergic receptors on cardiac and skeletal muscle, and increase the conversion of T4 to T3 in peripheral tissues. This mechanism explains why thyrotoxic symptoms (tachycardia, tremor, palpitations, anxiety) closely resemble beta-adrenergic hyperactivity and why beta-blockers (typically propranolol, though atenolol or metoprolol may be used) provide symptomatic relief independent of effects on thyroid hormone levels. Propranolol provides dual benefit in thyrotoxicosis: first, through classical beta-blockade of cardiac and skeletal muscle receptors, reducing heart rate and tremor; second, through the unique property of inhibiting peripheral D1 deiodinase activity (similar to PTU), thereby decreasing conversion of T4 to T3. This dual action makes propranolol particularly suitable in thyroid storm management.

Hypothyroidism Requiring Levothyroxine Replacement

• Autoimmune thyroiditis (Hashimoto's thyroiditis) represents the most common cause of hypothyroidism in iodine-sufficient regions, occurring in ~5-10% of the population with 5-10 fold higher prevalence in women; it results from autoreactive T cells and autoantibodies against TPO and thyroglobulin that progressively destroy thyroid follicles. Iodine deficiency remains the most common cause of hypothyroidism globally in iodine-deficient regions, particularly in Central Asia, Central Africa, and mountainous regions; severe iodine deficiency causes endemic cretinism characterized by intellectual disability and developmental abnormalities. Iatrogenic hypothyroidism follows thyroidectomy (surgical), radioactive iodine therapy for hyperthyroidism (dose-dependent, occurring in 10% at 1 year and increasing 2-3% annually thereafter), or antithyroid drug-induced hypothyroidism (particularly with methimazole, which may induce PTU-ANCA vasculitis when switched); radiation therapy to the neck for lymphoma or other malignancies damages thyroid tissue. Medications inducing hypothyroidism include amiodarone (contains high iodine load causing both hypo- and hyperthyroidism depending on individual iodine sufficiency), lithium (blocks iodine uptake and hormone release), interferon-alpha and interleukin-2 (direct thyroid toxicity), tyrosine kinase inhibitors (sunitinib, sorafenib), and checkpoint inhibitors (anti-PD-1, anti-CTLA-4) through immune-mediated thyroiditis. Central hypothyroidism from pituitary adenoma, hypopituitarism, or hypothalamic disease requires careful TSH-independent levothyroxine dosing. Secondary hypothyroidism may occur with pregnancy (increased TBG levels requiring dose adjustment), estrogen therapy, and nephrotic syndrome (urinary TBG loss).

Hyperthyroidism Requiring Antithyroid Agents

• Graves' disease (autoimmune thyroid stimulation via TSH receptor antibodies) accounts for 60-90% of hyperthyroidism and affects women 5-10 times more frequently than men, with peak incidence in 20-40 year age group; it may present acutely or insidiously and often coexists with other autoimmune conditions. Toxic multinodular goiter (TMNG) typically occurs in older patients with longstanding goiter and iodine sufficiency, resulting from autonomous hormone production by multiple thyroid nodules independent of TSH regulation. Toxic adenoma represents a single autonomously functioning nodule, typically in older patients with normal remaining thyroid tissue. Thyroiditis (subacute viral, postpartum, drug-induced) causes release of preformed thyroid hormones stored in colloid, producing transient hyperthyroidism unresponsive to antithyroid drugs (since no new synthesis occurs). Iodine-induced hyperthyroidism (Jod-Basedow phenomenon) occurs when iodine supplementation is given to iodine-deficient patients with autonomously functioning thyroid tissue, particularly in endemic iodine deficiency regions and with amiodarone therapy. TSH-secreting pituitary adenoma and human chorionic gonadotropin-secreting tumors (molar pregnancy, gestational trophoblastic disease, choriocarcinoma) cause secondary/tertiary hyperthyroidism.

Hypothyroidism Symptoms (Baseline for Levothyroxine Efficacy Assessment)

• Fatigue and decreased mental clarity result from reduced metabolic rate (20-40% decrease), diminished mitochondrial oxidative capacity, and impaired cerebral blood flow; patients characteristically report need for excessive sleep, slowed cognitive processing, and memory impairment. Cold intolerance reflects decreased thermogenesis and basal metabolic rate, with patients preferring warmer environments. Weight gain despite reduced appetite occurs through reduced metabolic rate exceeding decreased caloric intake; this weight gain characteristically involves myxedematous tissue deposition (glycosaminoglycan and mucopolysaccharide accumulation in subcutaneous tissues). Constipation develops from reduced gastrointestinal motility secondary to decreased beta-adrenergic tone and metabolic rate. Dry skin and coarse hair result from decreased sebaceous gland secretion and impaired hair follicle cycling; myxedematous changes produce characteristic facies with puffy face and periorbital edema. Menorrhagia and infertility occur through altered hypothalamic-pituitary-ovarian axis function and increased sex hormone-binding globulin (SHBG) levels. Depression and cognitive slowing reflect impaired noradrenergic and serotonergic neurotransmission, with severity correlating to degree of hypothyroidism.

Physical Examination Findings in Hypothyroidism

• Bradycardia (heart rate often 50-60 bpm or less) results from decreased cardiac sensitivity to catecholamines and reduced metabolic demands. Delayed relaxation phase of deep tendon reflexes (Achilles reflex most sensitive) produces characteristic "hung-up" reflexes reflecting slowed muscle contraction and relaxation kinetics. Myxedematous facies with periorbital puffiness, macroglossia (enlarged tongue), and delayed facial expressions. Macroglossia and hoarseness from laryngeal edema. Pale, cool, doughy skin with reduced perspiration.

Hyperthyroidism Symptoms (Baseline for Antithyroid Drug Efficacy Assessment)

• Tachycardia and palpitations represent the most common initial complaint, reflecting increased cardiac beta-adrenergic receptor number and sensitivity, increased myocardial oxygen consumption, and direct chronotropic effects of thyroid hormones on sinoatrial node. Anxiety, nervousness, and emotional lability develop from enhanced CNS catecholaminergic activity and increased norepinephrine turnover; this may progress to frank panic attacks or thyroid psychosis in severe cases. Tremor (characteristically fine, high-frequency resting tremor at 8-12 Hz) reflects enhanced beta-adrenergic activity in skeletal muscle. Heat intolerance with excessive diaphoresis results from increased metabolic rate and heat production; despite elevated core temperature, patients characteristically feel subjectively cold due to increased peripheral vasoconstriction. Weight loss despite increased appetite develops when increased metabolic rate substantially exceeds increased caloric intake; average weight loss of 10-15 pounds. Diarrhea or hyperdefecation from increased GI motility and beta-adrenergic tone. Ocular manifestations in Graves' disease including lid lag (slowed descent of upper eyelid when gaze shifts downward), lid retraction (exposure of sclera above iris), and stare, resulting from increased sympathetic tone and Müller's muscle contraction; exophthalmos (proptosis) represents infiltrative ophthalmopathy with orbital fat and extraocular muscle inflammation.

Physical Examination Findings in Hyperthyroidism

• Sinus tachycardia (resting heart rate often >90 bpm, sometimes >120 bpm) with decreased sleeping heart rate often remaining elevated. Systolic hypertension with widened pulse pressure reflecting increased cardiac output and decreased peripheral vascular resistance. Fine tremor best appreciated with hands extended and fingers spread. Warm, moist skin with hyperhidrosis. Thyroid bruit on auscultation (particularly in Graves' disease from increased blood flow). Thyroid goiter (diffuse and symmetric in Graves' disease, nodular in toxic multinodular goiter). Hyperreflexia with brisk deep tendon reflexes.

Thyroid-Stimulating Hormone (TSH) - Primary Screening Test

• TSH remains the most sensitive and specific screening test for primary thyroid dysfunction due to the steep sigmoidal relationship between free thyroid hormone concentrations and TSH secretion; TSH typically becomes abnormal before free hormone levels change significantly. Normal TSH range (0.5-5.0 mIU/L, though some laboratories use narrower range of 0.4-4.0 mIU/L) serves as the primary diagnostic reference. Elevated TSH (>5 mIU/L) suggests primary hypothyroidism and indicates need for levothyroxine initiation and titration to normal range. Suppressed TSH (<0.1 mIU/L) indicates either primary hyperthyroidism or excessive levothyroxine therapy. The TSH response is delayed—changes in TSH typically lag free hormone changes by 4-6 weeks due to slow pituitary response kinetics; therefore, TSH should be rechecked 6-8 weeks after levothyroxine dose changes and 6-12 weeks after antithyroid drug initiation. TSH sensitivity: approximately 95% for primary hypothyroidism and 99% for primary hyperthyroidism; TSH has approximately 97% sensitivity for detecting clinically significant thyroid dysfunction.

Levothyroxine — toxicity is iatrogenic thyrotoxicosis

• Cardiac: over-replacement upregulates myocardial beta-receptors and shortens atrial refractoriness, producing atrial fibrillation (especially in patients over 60) and precipitation of angina or infarction in occult CAD. The American Thyroid Association advises a reduced starting dose with slow upward titration in the elderly and in known coronary disease rather than full weight-based replacement.
• Skeletal: chronic TSH suppression accelerates osteoclastic bone turnover, lowering bone mineral density in postmenopausal women.
• Boxed warning: thyroid hormone must not be used for obesity or weight loss; at supraphysiologic doses, especially with sympathomimetic amines, it causes serious and potentially fatal toxicity.
• Contraindication/sequence error: giving levothyroxine before glucocorticoids in coexisting adrenal insufficiency accelerates cortisol clearance and can precipitate adrenal crisis — steroids first.
• No true antidote: massive ingestion is managed supportively with beta-blockade; cholestyramine interrupts enterohepatic recirculation of T4, and activated charcoal is useful only very early.

Thionamides (methimazole, PTU)

• Agranulocytosis: rare (well under 1%) and largely unpredictable; with methimazole the risk appears higher at daily doses above roughly 40 mg, whereas PTU risk is not clearly dose-related. It typically occurs within the first few months and is cross-reactive between the two drugs — do not simply switch agents. ATA recommends a baseline CBC with differential rather than routine surveillance counts; any fever or sore throat mandates stopping the drug and obtaining an immediate differential.
• PTU hepatotoxicity: FDA boxed warning for fulminant hepatocellular necrosis and liver failure requiring transplant, including in children. This is why methimazole is first-line except in the first trimester, thyroid storm, or methimazole intolerance. Methimazole injury, when it occurs, is typically cholestatic.
• Methimazole embryopathy: aplasia cutis, choanal and esophageal atresia with first-trimester exposure.
• PTU-associated ANCA (anti-MPO) vasculitis and drug-induced lupus; also rash, urticaria, and an antithyroid arthritis syndrome.
• Overtreatment causes drug-induced hypothyroidism with goiter from TSH rebound.

• Fever and sore throat on methimazole: the single best next step is to stop the drug and obtain a CBC with differential for agranulocytosis — not to reassure, not to add an antibiotic empirically without a count.
• Pregnancy sequencing: PTU is preferred in the first trimester (avoids aplasia cutis and choanal/esophageal atresia from methimazole embryopathy); many clinicians and most exam sources then switch to methimazole after the first trimester to limit PTU hepatotoxicity, though the current ATA pregnancy guideline notes the evidence for switching is inconclusive.
• PTU's two other niches: thyroid storm (it blocks peripheral D1 deiodinase, so T4→T3 conversion falls) and methimazole intolerance. Otherwise methimazole is first-line because of PTU's boxed hepatic failure warning.
• Storm drug order: beta-blocker (propranolol) and thionamide first, glucocorticoid, and iodine (Lugol's/SSKI) at least an hour after the thionamide — iodine given first supplies substrate for new hormone synthesis in an unblocked gland and can worsen thyrotoxicosis.
• Levothyroxine absorption: empty stomach, 30–60 minutes before breakfast, separated by about 4 hours from calcium, iron, and bile acid sequestrants; PPIs and malabsorption raise requirements. A patient with a persistently high TSH on a seemingly adequate dose is nonadherent or mis-timing the dose until proven otherwise.
• Pregnancy in a hypothyroid woman: estrogen raises TBG, so levothyroxine requirement rises early — the ATA advises increasing the dose promptly (a commonly taught maneuver is two extra tablets per week) and rechecking TSH against trimester-specific ranges.
• Recheck TSH about 6–8 weeks after any dose change — the pituitary lags; earlier testing produces spurious adjustments.
• Distractors to avoid: TSH is useless for titration in central hypothyroidism — follow free T4. Thionamides do nothing for thyroiditis (preformed hormone release, low radioiodine uptake); treat with beta-blockade. And in suspected myxedema coma, give empiric IV glucocorticoid alongside IV levothyroxine until adrenal insufficiency is excluded.