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.
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.
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)
5 pairs found. Sorted from critical to minor.
Mechanism
Thyroid hormones accelerate catabolism of vitamin K-dependent clotting factors. When levothyroxine is initiated or its dose is raised in a patient becoming euthyroid, warfarin effect increases – INR rises and the warfarin dose must be reduced. Conversely, hypothyroidism or a tyroxine dose reduction lowers INR.
Management
Check INR at 1, 2, and 4 weeks after starting or adjusting levothyroxine. A 10–25 % reduction in the warfarin dose is usually needed when the patient becomes euthyroid.
Sources
Mechanism
Кальций связывает левотироксин в просвете и снижает его абсорбцию на 20–25%.
Management
Разнести прием минимум на 4 часа. Левотироксин утром натощак, кальций – через 4 часа или вечером.
Mechanism
Levothyroxine sodium requires gastric acidity to dissolve. Omeprazole raises gastric pH – thyroxine absorption falls by 20–30 %. Clinically this presents as rising TSH and failure to achieve targets in some patients.
Management
Take levothyroxine strictly on an empty stomach, 30–60 minutes before food and other drugs. Take the PPI 2 hours later or in the evening. Check TSH at 6–8 weeks after starting omeprazole and, if needed, increase the levothyroxine dose by 12.5–25 µg.
Sources
Mechanism
Amoxicillin can interfere with intestinal absorption of levothyroxine when taken simultaneously – clinically insignificant when administration is separated.
Management
Take levothyroxine and amoxicillin at least 2 hours apart.
Sources
Mechanism
Hypothyroidism by itself raises . Achieving euthyroidism with levothyroxine reduces statin requirements. Clinical significance is mainly a target-dose adjustment after TSH normalization.
Management
Repeat the lipid profile 6–8 weeks after euthyroidism is achieved and adjust atorvastatin dose if needed.
Sources
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 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).