Levothyroxine (T4): a seven-day half-life, and why absorption decides the dose

The most prescribed hormone in the country, and a drug whose dose is decided less by pharmacology than by how much of the tablet actually gets absorbed. Six weeks to steady state, and a long list of things that interfere.

A prohormone, and what that implies

Established clinical use Levothyroxine is synthetic thyroxine, and thyroxine is not the active hormone. It circulates bound to thyroxine-binding globulin, albumin and transthyretin, and the deiodinase enzymes in peripheral tissue convert a fraction of it to triiodothyronine, which is what binds the receptor. Replacing T4 therefore relies on the conversion step working, which in most people it does.

Its half-life is about a week, and that single number explains most of how it is used. Once-daily dosing produces a nearly flat level. A missed dose barely registers. And a change takes about five half-lives to settle, which is why the conventional interval before rechecking is six weeks — a panel drawn at three weeks is measuring a level still moving, and adjusting on it is how people end up oscillating.

The therapeutic index is narrow enough that formulation matters. Different products and different generics are not always superimposable in absorption, and switching between them without rechecking is a recognised source of drift.

Absorption is the variable

Established clinical use Levothyroxine is absorbed in the small intestine and the fraction absorbed is modest and easily reduced. Food does it. Coffee does it, specifically and measurably. Calcium and iron supplements bind it. Proton pump inhibitors and other acid suppression reduce it. Coeliac disease, atrophic gastritis and H. pylori reduce it. This is why the instruction is an empty stomach with water, thirty to sixty minutes before anything else, and why "take it consistently" matters more than which hour you pick.

Consistency is the actionable version of all of that. A person who takes it with coffee every day is on a stable, if lower, absorbed dose; a person who takes it with coffee some days is producing their own variability and then attributing it to the thyroid. The most common reason a dose "stopped working" is a change in what surrounds it.

Reading the panel, and one thing testosterone does to it

Dosing is conventionally titrated against TSH, and the argument about that is genuine rather than fringe: a proportion of people on levothyroxine with a TSH squarely in range continue to report symptoms, and whether that reflects inadequate tissue T3, an unrelated cause, or the limits of a population reference interval applied to an individual is not settled. Off-label or community practice Combination therapy with liothyronine is the intervention that argument usually leads to, and the randomised evidence for it has mostly not shown consistent benefit over T4 alone.

One finding on this panel is specific to this site’s readers and is not thyroid disease. Established clinical use Testosterone lowers thyroxine-binding globulin, so a man on testosterone therapy can show a low total T4 with entirely normal free hormones and a normal thyroid. Reading totals rather than free hormones in that situation generates alarm and sometimes generates a prescription. The thyroid panel page covers which values to read and why.

Over-replacement is the risk that accumulates quietly: a suppressed TSH sustained over years is associated with reduced bone density and with atrial fibrillation, particularly in older people. None of that is a page’s call to make. Dose, formulation and target belong with the clinician who prescribes, reading your own report against the interval your own lab printed.

How long one dose lasts

Try other intervals on the half-life and steady-state calculator, which runs the same function against whatever cadence you type.

The dosing rows the app records

Established clinical use Reproduced from the app’s reference so you can see what it holds, not as a recommendation. Which row applies to a particular person, if any, is a clinical decision this page does not make — and rows describing supraphysiological or post-cycle use are filtered out before this table is built, so what you see here is a subset.

What the app monitors alongside it

The panel below is the app’s own monitoring note for Levothyroxine (T4), verbatim. The analytes in it that have a page here are linked; those pages cover what each one measures, which assay produced it and how to read a trend.

• Free versus total testosterone, and the three ways labs get the free number
• Thyroid panel

Drawbacks and risks the app records

From the app’s own entry. Note how many are about absorption and conversion rather than about the drug: that is an accurate reflection of where the problems live.

• Requires conversion to T3 — poor converters see suboptimal response
• Slower onset than T3
• TSH-based dosing may leave patients symptomatic in low-normal range
• Generic vs brand differences in absorption matter
• Absorption affected by food, coffee, calcium, iron
• Doesn't address Reverse T3 issues

Interaction rules that name Levothyroxine (T4)

From the app’s interaction data, filtered so no rule naming a compound this site does not publish appears. Not exhaustive, and not a safety clearance: a combination that is not listed is one nobody has documented here, which is not the same as one that is fine. The combination checker has the rest.

Questions people actually ask

How long before labs mean anything after a change?

About six weeks. Five half-lives at a week each is five weeks to a settled level, and TSH lags the free hormones by a further stretch. Drawing earlier measures a system still moving.

Does it matter which brand or generic?

It can. The therapeutic index is narrow and absorption is not identical across products, so switching is a reason to recheck at six weeks rather than assume equivalence. That is a pharmacy and prescriber conversation.

Why does my total T4 look low on testosterone therapy?

Because testosterone lowers thyroxine-binding globulin, and total T4 measures bound plus free. The free hormones are the ones to read. This is a binding-protein effect, not thyroid disease, and it is covered on the thyroid panel page.

Is T4 alone enough?

For most people the conversion step works and it is. A minority report persistent symptoms with a normal TSH, and that is where the combination-therapy argument comes from — an argument the randomised evidence has not settled in favour of either side.

Where the load-bearing numbers come from

Named rather than linked. Publisher URLs move, and a citation that resolves to a 404 two years from now is worse than one you can search for by name — every entry below is findable from the title and year alone.