Thyroid hormones
ATC code: H03AA01 (Levothyroxine sodium)
Brand names
Synthroid, Levoxyl, Tirosint, Unithroid
Synthetic analogue of endogenous thyroxine (T4). Deiodinated in tissues to the biologically active triiodothyronine (T3), which binds nuclear thyroid hormone receptors and regulates genes controlling basal metabolism, growth, and development. Affects carbohydrate, lipid, and protein metabolism, the cardiovascular system, CNS, and skeleton.
First line
Lifelong replacement is required after total thyroidectomy. In thyroid cancer, TSH suppression to below 0.1 mU/L is the target in high-risk patients (per AJCC and staging) and 0.1–0.5 mU/L in intermediate risk. After subtotal resection for benign disease, the goal is euthyroidism with the same TSH target as for hypothyroidism.
Sources
First line
Replacement therapy in primary hypothyroidism — first-line and sole therapy per 2014 and . Starting dose in young adults is 1.6 mcg/kg/day. In elderly and patients with ischemic heart disease, start at 12.5–25 mcg and titrate slowly. TSH target is 0.4–4.0 mU/L, with an upper limit up to 6 mU/L acceptable in the elderly. T4+T3 combinations are not routinely recommended.
Individual decision
Subclinical hypothyroidism (TSH 4.5–10 mU/L, normal T4) may be treated selectively. recommends therapy when TSH exceeds 10 mU/L, in patients with TPO antibodies and hypothyroid symptoms, and in women who are pregnant or planning pregnancy. Asymptomatic elderly patients are generally not treated because of iatrogenic thyrotoxicosis risk.
The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.
Not recommended
Using levothyroxine for weight loss in euthyroid patients is not supported by international guidelines and is potentially harmful. Iatrogenic thyrotoxicosis increases the risk of atrial fibrillation, osteoporosis, and cardiomyopathy without producing sustained weight loss.
Calcium binds levothyroxine in the gut, reducing absorption by 20–25%. Take levothyroxine in the morning on an empty stomach; calcium 4 hours later or in the evening. Check TSH if dosing schedule changes.
Source: ATA: Guidelines for the Treatment of Hypothyroidism (2014)
Iron binds levothyroxine in the gut. Absorption drops, TSH may rise. Separate by at least 4 hours: levothyroxine in the morning on empty stomach; iron 4 hours later or in the evening.
Source: ATA: Guidelines for the Treatment of Hypothyroidism (2014)
6 pairs found. Sorted from critical to minor.
Mechanism
Thyroid hormones accelerate catabolism of vitamin K-dependent clotting factors. When levothyroxine starts or its dose increases, warfarin sensitivity rises and INR climbs within 1–4 weeks.
Symptoms
Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool. Severe cases include gastrointestinal or intracranial haemorrhage.
Management
When levothyroxine starts or is escalated, check INR at 1, 2, and 4 weeks and titrate warfarin (typically a 10–25% dose reduction). Once thyroid function stabilises, warfarin requirements stabilise too.
Sources
Mechanism
Calcium binds levothyroxine in the gut as insoluble complexes, reducing absorption by 20–25%.
Symptoms
Return of hypothyroidism: fatigue, weight gain, cold intolerance, dry skin, low mood. In thyroid cancer, loss of TSH control.
Management
Take levothyroxine in the morning on empty stomach, 30–60 minutes before food and other drugs. Take calcium carbonate 4 hours after levothyroxine (e.g. at lunch or in the evening). Check TSH 6–8 weeks after starting calcium.
Mechanism
Proton pump inhibitors raise gastric pH, reducing levothyroxine absorption. TSH may rise 2–4 weeks after starting omeprazole.
Symptoms
Return of hypothyroidism: fatigue, weight gain, cold intolerance, dry skin, low mood. In thyroid cancer (TSH suppression needed), loss of control.
Management
Take levothyroxine 30–60 minutes before food and omeprazole. Check TSH 6–8 weeks after starting or stopping omeprazole. If TSH rises, increase levothyroxine by 12.5–25 mcg.
Sources
Mechanism
Amoxicillin taken concurrently may slightly impair intestinal levothyroxine absorption. With separated dosing, the effect is clinically negligible.
Symptoms
The combination usually causes no specific symptoms. Each drug's individual side effects remain.
Management
Take levothyroxine in the morning on an empty stomach, 30–60 minutes before food and other medications. Take amoxicillin 4 hours later or at lunch. For short amoxicillin courses (5–10 days), no special TSH monitoring needed.
Sources
Mechanism
Ascorbic acid raises gastric acidity and improves levothyroxine absorption in patients with atrophic gastritis or hypochlorhydria. In healthy patients the effect is minimal.
Symptoms
The combination usually causes no specific symptoms. Each drug's individual side effects remain.
Management
In patients with confirmed atrophic gastritis or PPI intolerance, ascorbic acid 500 mg with levothyroxine improves absorption. In healthy patients, no specific adjustment is needed. Monitor TSH on the usual schedule.
Mechanism
Hypothyroidism itself raises . Restoring euthyroid status on levothyroxine reduces statin need. No direct pharmacokinetic interaction.
Symptoms
The combination usually causes no specific symptoms. Each drug's individual side effects remain.
Management
On starting levothyroxine in patients on stable atorvastatin, check lipids 6–8 weeks after TSH normalises. If drops below target, atorvastatin can be reduced. In newly diagnosed hypothyroidism with hypercholesterolaemia, delay statin start until euthyroid status is achieved.
Sources
Opens the checker prefilled with this drug. Pick the second one from your regimen.
Direct links to regulator labels. Open in a new tab.
FDA category A. Safe and required in pregnancy for hypothyroid women. Inadequate replacement is associated with miscarriage, preterm birth, and impaired cognitive development of the child.
Compatible with breastfeeding. The amount transferred to milk is physiologically insignificant.
Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.
Levothyroxine sodium is evaluated for the following indications with varying evidence strength: Post-thyroidectomy state (evidence tier A), Primary hypothyroidism (evidence tier A), Subclinical hypothyroidism (evidence tier B). See the full indication matrix with dosing and citations above on this page.
Common side effects of Levothyroxine sodium (≥ 1 in 100): Symptoms of overdose at excessive doses: tachycardia, tremor, weight loss, sweating, nervousness, Headache. See the Safety section for uncommon and serious reactions.
FDA category A. FDA category A. Safe and required in pregnancy for hypothyroid women. Inadequate replacement is associated with miscarriage, preterm birth, and impaired cognitive development of the child.
Compatible with breastfeeding. The amount transferred to milk is physiologically insignificant.
Levothyroxine sodium is contraindicated in: Untreated thyrotoxicosis; Acute myocarditis; Acute myocardial infarction; Untreated adrenal insufficiency; Hypersensitivity. Full list in the Safety section.
levothyroxine is bioidentical to endogenous thyroxine. Herbal products contain no active hormone and cannot compensate for hypothyroidism.
replacement for primary hypothyroidism is lifelong. Discontinuation leads to symptom return within weeks and risks myxedema coma with prolonged decompensation.
in euthyroid patients it does not produce sustained weight loss and is harmful (see the 'weight loss in euthyroid patients' indication).