Levothyroxine
Synthetic T4 thyroid hormone replacement — the most prescribed thyroid drug in the United States. Restores normal thyroid hormone levels in hypothyroidism. Has a narrow therapeutic index requiring careful, individualized dosing management.
How It Works
Levothyroxine is a synthetic form of thyroxine (T4), the principal hormone secreted by the thyroid gland. After absorption, T4 is converted to the more active triiodothyronine (T3) in peripheral tissues. T3 binds to nuclear thyroid hormone receptors and regulates gene expression throughout the body — controlling metabolic rate, heart rate, body temperature, protein synthesis, and development.
In hypothyroidism, the thyroid produces insufficient T4. Levothyroxine replacement restores circulating T4 and, through conversion, T3 levels to the normal range, relieving symptoms of hypothyroidism such as fatigue, cold intolerance, weight gain, and cognitive slowing. TSH (thyroid-stimulating hormone) — the pituitary signal that drives thyroid production — is used to monitor adequacy of replacement, with a target TSH range individualized by clinical context.
Common Uses
- Hypothyroidism (underactive thyroid) — primary and secondary
- TSH suppression therapy — after thyroid cancer surgery to reduce the risk of recurrence
- Myxedema coma — severe hypothyroidism (IV formulation used in hospital)
- Goiter suppression in select cases
Common Side Effects
At correct replacement levels, levothyroxine is generally well tolerated with minimal side effects. Most side effects reflect over-replacement (too much thyroid hormone):
- Palpitations, rapid or irregular heartbeat (tachycardia, arrhythmia)
- Anxiety, nervousness, irritability, insomnia
- Tremor (fine hand tremor)
- Unintended weight loss, increased sweating
- Heat intolerance, diarrhea
- Bone loss (osteoporosis) with chronic over-replacement, especially in postmenopausal women
Under-replacement symptoms mirror hypothyroidism: fatigue, weight gain, cold intolerance, constipation, and depression.
Warnings & Precautions
NOT FOR WEIGHT LOSS: Levothyroxine must not be used to treat obesity or weight gain in people with normal thyroid function. Doses beyond replacement needs can cause serious or life-threatening cardiac toxicity.
- Narrow therapeutic index: Small differences in blood levels cause clinically significant under- or over-treatment. Maintain consistency in brand and formulation — do not switch between brands or generic manufacturers without re-checking TSH.
- Calcium and iron interactions: Separate levothyroxine from calcium supplements, iron supplements, antacids, and cholestyramine by at least 4 hours — these drugs bind levothyroxine in the gut and reduce absorption
- Coffee: Even black coffee reduces absorption — take levothyroxine with water only, 30–60 minutes before eating or drinking anything else
- Caution in patients with cardiovascular disease, osteoporosis, or adrenal insufficiency (treat adrenal insufficiency before starting levothyroxine)
- Many drugs affect levothyroxine levels — warfarin, digoxin, antiepileptics, certain antacids — inform all providers you are on thyroid replacement
Common Pill Imprints
Synthroid tablets are color-coded by strength — a consistent visual identifier even when the imprint is small. Generics vary by manufacturer. All are round tablets.
| Imprint | Strength | Manufacturer | Description |
|---|---|---|---|
| SYNTHROID 50 | 50 mcg | AbbVie (Synthroid) | White round tablet |
| SYNTHROID 100 | 100 mcg | AbbVie (Synthroid) | Yellow round tablet |
| SYNTHROID 125 | 125 mcg | AbbVie (Synthroid) | Brown round tablet |
| M L 5 | 50 mcg | Mylan (generic) | White round tablet |
Frequently Asked Questions
No — take levothyroxine on an empty stomach with water only, at least 30–60 minutes before eating or drinking coffee. Even black coffee can reduce absorption. Consistency in how you take it each morning is as important as the specific timing.
Levothyroxine has a narrow therapeutic index — small differences in absorbed levels shift patients between optimal and over- or under-replaced. Switching brands or between generic manufacturers without rechecking TSH can cause clinically meaningful changes. Stick to the same brand and formulation whenever possible.
Separate them by at least 4 hours. Calcium carbonate, iron, antacids, and some fiber supplements bind levothyroxine in the gut and significantly reduce absorption, leading to lower-than-expected thyroid levels and under-treatment.
It should not be. Levothyroxine is labeled only for hypothyroidism and TSH suppression in thyroid cancer. Using it for weight loss in people with normal thyroid function can cause serious cardiovascular harm including arrhythmias and bone loss.