💊 Levothyroxine (Renthyroid)

✅ What it is used for

• Replacement or supplemental therapy in hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis
• Primary hypothyroidism from functional deficiency, primary atrophy, partial or total absence of the gland, or the effects of surgery, radiation, or drugs, with or without goiter
• Secondary (pituitary) and tertiary (hypothalamic) hypothyroidism
• Pituitary TSH suppression in the treatment or prevention of euthyroid goiters, including thyroid nodules, subacute or chronic lymphocytic thyroiditis (Hashimoto's), and multinodular goiter
• Management of thyroid cancer

🧪 Active ingredient

The product is a desiccated porcine thyroid extract containing T4 (levothyroxine) and T3 (liothyronine); each 60 mg of labeled thyroid content provides 38 mcg T4 and 9 mcg T3. Thyroid hormones enhance oxygen consumption in most body tissues, increase the basal metabolic rate, and affect carbohydrate, lipid, and protein metabolism, with particular importance for central nervous system development.

🌿 Benefits

• Replaces missing thyroid hormone in hypothyroidism
• Normalizes T4 and TSH levels with adequate therapy
• Suppresses TSH to treat or prevent certain goiters
• Used as ancillary therapy in thyroid cancer

📝 How to use

• Taken by mouth.
• The dose is individualized by your doctor according to your response and laboratory results; do not change the dose on your own.
• Treatment usually starts at low doses that are increased gradually depending on cardiovascular status.
• Tell your doctor if you take biotin or biotin-containing supplements, as they can interfere with thyroid tests; you may be asked to stop biotin at least 2 days before TSH/T4 testing.
• Clinical and laboratory evaluation (T4 and TSH) is needed within the first weeks to adjust the dose.

💊 Dosage forms

• Tablets in strengths: 1/4 grain (15 mg), 1/2 grain (30 mg), 3/4 grain (45 mg), 1 grain (60 mg), 1 1/4 grain (75 mg), 1 1/2 grain (90 mg), 2 grain (120 mg), 3 grain (180 mg)

🔄 Drug interactions

• Oral anticoagulants: thyroid hormones increase catabolism of vitamin K-dependent clotting factors; anticoagulant dose reduction is often required; close monitoring needed
• Insulin or oral hypoglycemics: requirements may increase when thyroid therapy starts; monitor closely
• Cholestyramine or colestipol: bind T4 and T3 in the intestine and impair absorption; separate by 4 to 5 hours
• Estrogens/oral contraceptives: increase TBG; patients without functioning thyroid may need a higher thyroid dose
• Biotin: may interfere with TSH, T4, and T3 immunoassays

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👨‍⚕️ For healthcare professionals

Mechanism of action

Exogenous thyroid hormone in excess of the normal gland's secretion suppresses endogenous secretion in euthyroid individuals via TSH/TRH feedback. Hormones enhance oxygen consumption, basal metabolic rate, and carbohydrate, lipid, and protein metabolism. About 80% of peripheral T3 derives from monodeiodination of T4. T3 is approximately four times as potent as T4 on a microgram basis.

Labeled dosing

Per label for adult hypothyroidism: usual starting dose 30 mg with increments of 15 mg every 2 to 3 weeks; lower start of 15 mg/day in long-standing myxedema or suspected cardiovascular impairment. Most patients require 60 to 120 mg/day; failure to respond to 180 mg suggests non-compliance or malabsorption. Readjust within the first four weeks after clinical and laboratory evaluation (T4, TSH). Dosing is individualized at the prescriber's discretion.

• Myxedema coma (emergency): intravenous route preferred; levothyroxine 400 mcg initially, then 100 to 200 mcg IV daily, with correction of electrolytes and infection and routine corticosteroids
• Thyroid cancer: TSH should be suppressed to low or undetectable levels as ancillary therapy with radioactive iodine
• T3 may be preferred over T4 during radio-isotope scanning or when impaired peripheral conversion is suspected
• Intramuscular administration is not advisable due to poor absorption
• Acute massive overdose: reduce GI absorption, supportive care, antiadrenergics such as propranolol; cholestyramine and glucocorticoids may be adjunctive

🎓 Key points for students

• Desiccated porcine thyroid containing both T4 and T3, not pure levothyroxine
• T3 is about 4x as potent as T4 mcg for mcg
• 60 mg provides 38 mcg T4 and 9 mcg T3
• About 80% of peripheral T3 comes from T4 deiodination
• Contraindications: uncorrected adrenal insufficiency, untreated thyrotoxicosis, hypersensitivity
• Estrogens raise TBG, increasing dose needs in patients without functioning thyroid
• Cholestyramine impairs absorption: separate 4 to 5 hours
• Biotin interferes with TSH/T4/T3 immunoassays