Synthroid (Levothyroxine) 25‑300 mcg Colour-Coded Tablets: Prescription Synthetic T4 for Hypothyroidism

Information last reviewed: May 2026 — for educational purposes only.

Synthroid (levothyroxine sodium) is a synthetic form of thyroxine (T4) — the primary thyroid hormone produced by the thyroid gland. It is the most prescribed medication in the United States, with tens of millions of prescriptions annually, reflecting the high prevalence of hypothyroidism. By replenishing T4 levels, levothyroxine restores normal metabolic rate, energy metabolism, body weight regulation, cardiac function, cognitive clarity, skin and hair health, and the many other physiological processes regulated by thyroid hormone. Once absorbed from the GI tract, T4 circulates bound to thyroid-binding globulin (TBG) and is peripherally deiodinated to the more metabolically active T3 (triiodothyronine) in target organs — meaning the body converts T4 to T3 as needed, self-regulating active hormone availability.

Levothyroxine has a long half-life of approximately 7 days, allowing once-daily dosing and a stable hormonal effect over time. However, its narrow therapeutic index means that small differences in dose or bioavailability produce meaningful changes in TSH — potentially causing symptoms of hypothyroidism (too little) or hyperthyroidism (too much). Different brands and even different generic manufacturers produce levothyroxine with slightly different bioavailability profiles, which is why endocrinologists and the FDA recommend that patients remain on the same brand or generic manufacturer rather than switching each refill.

What Is Levothyroxine?

Levothyroxine sodium is identical in structure to the T4 secreted naturally by the human thyroid gland. The standard treatment for primary hypothyroidism (underactive thyroid gland) is T4-only replacement because peripheral tissues contain sufficient deiodinase activity to convert T4 to T3. In most patients, this provides an adequate supply of both T4 and T3 without needing separate T3 supplementation (liothyronine, Cytomel). For patients who continue to experience hypothyroid symptoms despite normal TSH on levothyroxine alone, some endocrinologists consider combination T4+T3 therapy — though evidence for routinely adding liothyronine is mixed. Standard monitoring on stable levothyroxine therapy is TSH measurement every 6–12 months; more frequent monitoring is needed in pregnancy (TSH requirements change), dose adjustments, and newly initiated therapy (recheck TSH 6–8 weeks after any dose change).

Prescription Status

Levothyroxine is prescription-only in the United States. It should not be used for weight loss in patients with normal thyroid function — doing so causes iatrogenic hyperthyroidism with risks of cardiac arrhythmias, bone loss, and tremor. Diagnosis of hypothyroidism requires laboratory confirmation (TSH above reference range, often with low free T4) before starting treatment.

Strengths and Available Forms

Levothyroxine sodium tablets are available in 12 strengths, each colour-coded to facilitate dose identification:

• 25 mcg — orange
• 50 mcg — white
• 75 mcg — violet/purple
• 88 mcg — mint green (olive)
• 100 mcg — yellow
• 112 mcg — rose
• 125 mcg — brown
• 137 mcg — turquoise
• 150 mcg — blue
• 175 mcg — lilac/lavender
• 200 mcg — pink
• 300 mcg — green

Typical starting dose in healthy adults under 50: 1.6 mcg/kg/day. Starting doses are lower in elderly patients, those with cardiac disease, and pregnant women (individualized). Doses are adjusted in 12.5–25 mcg increments with TSH rechecked 6–8 weeks after each change.

Price of Synthroid and Generic Levothyroxine

Generic levothyroxine is widely available at very low cost ($4–10/month at discount programmes). Brand-name Synthroid (Abbott) is significantly more expensive ($30–80/month without insurance). Both are clinically effective; the important factor is consistency — patients should not switch between brands each refill. Insurance typically covers generic levothyroxine; prior authorisation may be required for brand-name Synthroid.

Frequently Asked Questions

Why must levothyroxine be taken on an empty stomach and why does it matter?

Levothyroxine absorption from the GI tract is incomplete (approximately 40–80% of the dose is absorbed) and is significantly reduced by food, coffee, calcium, iron, antacids, bile acid sequestrants, and certain medications. Taking levothyroxine in the morning on an empty stomach — ideally 30–60 minutes before breakfast with plain water — maximises and stabilises its absorption. If taken with food, coffee, or calcium-containing supplements, absorption can be reduced by 30–40%, effectively under-dosing the patient and resulting in persistently hypothyroid TSH levels despite adequate prescription strength. Some patients who find fasting administration difficult may take levothyroxine at bedtime (several hours after the last meal), which also provides good absorption — but the administration time must be kept consistent.

What are the interactions between levothyroxine and calcium, iron, and antacids?

Calcium carbonate, calcium citrate, ferrous sulfate (iron), aluminium/magnesium antacids, sucralfate, and proton pump inhibitors all reduce levothyroxine absorption through different mechanisms (binding, pH alteration, reduced dissolution). These supplements and medications must be taken at least 4 hours apart from levothyroxine — not simultaneously. Among the most common causes of persistent hypothyroidism despite apparently adequate levothyroxine doses is concurrent calcium or iron supplement use at the same time as levothyroxine. Patients starting or stopping calcium/iron should have TSH rechecked 6 weeks later.

What happens if too much levothyroxine is taken (hyperthyroidism symptoms)?

Excessive levothyroxine causes iatrogenic hyperthyroidism — characterised by palpitations, tachycardia/AF risk, fine tremor, heat intolerance, sweating, weight loss, anxiety, insomnia, and diarrhoea. Chronically excessive levothyroxine also causes accelerated bone loss (osteoporosis risk) and increased cardiovascular risk. Subclinical hyperthyroidism (suppressed TSH with normal-high T4) is associated with AF and fractures even without overt hyperthyroid symptoms. This is why TSH target range is individualised: the typical target TSH for standard hypothyroidism is 0.5–2.5 mIU/L; pregnancy (first trimester: <2.5); thyroid cancer surveillance (suppressed to 0.1–0.5 or <0.1 depending on risk).

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