Thyroxine and thyroid hormone replacement — patient guide

Your thyroid normally makes two hormones — T4 (thyroxine) in larger amounts, and a smaller amount of T3 (the active form). Body tissues convert T4 to T3 locally using deiodinase enzymes (which depend on selenium, zinc, and iron as co-factors). T3 is what actually engages thyroid receptors and drives metabolism, mood, temperature regulation, gut motility, cognition, and a long list of background functions.

Levothyroxine is synthetic T4 — the precursor. You give the body the substrate and trust the deiodinases to do the conversion. This works for most people. It’s the simplest, most predictable approach, and the long half-life produces a smooth steady state.

Liothyronine is synthetic T3 — the active hormone, given directly. Because it’s short-acting, dosing is split through the day. It’s specialist territory and not usually first-line.

Desiccated thyroid extract (NDT) is dried porcine thyroid. It contains both T4 and T3 in a fixed ratio of roughly 4:1, set by porcine biology (the human thyroid produces closer to 14:1). The argument for NDT is that some people convert T4 to T3 less effectively and feel better with some direct T3 in the mix. The argument against is that the fixed ratio doesn’t suit everyone, the porcine source has more day-to-day variability than synthetic, and the randomised-trial evidence comparing NDT to levothyroxine is mixed and short in duration.

The integrative reframe here is mechanism-first. If T3 isn’t reaching tissues despite adequate T4, the question is why — nutrient co-factors for the deiodinases (selenium, zinc, iron), reverse-T3 in physiological stress states, gut absorption, or genuine genetic variation in conversion enzymes. Sometimes the answer is to add T3 or switch to NDT; sometimes the answer is upstream of the medicine.

Most “side effects” of thyroxine are actually signs of under- or over-replacement. True intolerance to the molecule is uncommon; intolerance to a particular brand or excipient is real but uncommon.

Signs the dose may be too low (under-replacement)

• Persistent fatigue, brain fog, low mood
• Cold intolerance
• Constipation
• Dry skin and hair, hair loss
• Weight gain or difficulty losing weight
• Slow pulse
• Heavy menstrual bleeding

Signs the dose may be too high (over-replacement / iatrogenic thyrotoxicosis) — the more concerning end (the TRUST trial NEJM 2017 is one of the clearest demonstrations that aggressive replacement of mild subclinical hypothyroidism in older adults didn’t produce symptom benefit and risked over-treatment):

• Palpitations, racing heart, atrial fibrillation
• Hand tremor
• Heat intolerance, increased sweating
• Anxiety, insomnia, irritability
• Unintentional weight loss
• Loose stools
• Bone loss over time (especially in postmenopausal women on TSH-suppressed dosing)
• Cardiovascular events in older patients with underlying ischaemic heart disease

Over-replacement matters more than under-replacement in adults over 65, anyone with atrial fibrillation, anyone with known ischaemic heart disease, and postmenopausal women (bone density). “Start low, go slow” exists for these groups.

Brand-related symptoms

• Persistent symptoms after a pharmacy substitution from one brand to another, despite the milligram number being the same — patient reports are common and bioequivalence by regulatory standard does not always translate to felt-equivalence. Re-check TSH at 6-8 weeks if a switch has happened; consider asking for a return to the original brand.

Rare but worth knowing

• True allergy to a tablet excipient (lactose in some formulations, dye in some colours). Switching brand or formulation usually resolves it.
• Severe pre-existing cardiac disease — if a low starting dose triggers chest pain or worsening angina, hold the next dose and contact us. Endocrinology and cardiology shared care is appropriate.

This is the section where most of the “I take it but my TSH won’t settle” problems live. Levothyroxine absorption is fragile.

Take 30-60 minutes apart from these (longer is better):

• Coffee — significant reduction; 60 minutes minimum.
• Food — any food drops absorption.
• Calcium (tablets, antacids like Mylanta, fortified plant milks, dairy with breakfast).
• Iron supplements.
• Multivitamins with minerals (which contain calcium, iron, magnesium, and sometimes zinc).
• Magnesium supplements.

Separate by at least 4 hours from these (stronger blockers):

• Proton-pump inhibitors (PPIs) — esomeprazole/Nexium, pantoprazole/Somac, omeprazole/Losec. Long-term PPI use can raise levothyroxine requirement; if you’re starting or stopping a PPI, plan a TSH re-check 6-8 weeks later. (See Liwanpo & Hershman 2009 for the absorption-interaction review.)
• Bile-acid sequestrants — cholestyramine, colestipol.
• Phosphate binders — sevelamer, lanthanum (used in chronic kidney disease).
• Sucralfate.
• High-fibre cereals at the same time — modest but real reduction.
• Significant soy intake (soy milk daily, large amounts of soy protein) — moderate dietary soy is fine; sustained high intake reduces absorption.

Medicines that change how fast your body uses thyroxine (alter requirement)

• Oestrogen — combined oral contraceptive pill, oral menopausal hormone therapy (MHT/HRT). Raises thyroxine-binding globulin and commonly increases requirement by ~25%. Transdermal oestrogen has less effect. Plan a TSH re-check 6-8 weeks after starting or stopping.
• Androgens — testosterone replacement. Tends to reduce requirement.
• Carbamazepine, phenytoin, rifampicin, St John’s wort — induce thyroxine metabolism, increasing requirement.
• Amiodarone — complex effects on thyroid; specialist territory.
• Lithium — can precipitate or worsen hypothyroidism.

Food. No “thyroid diet.” Normal varied AU eating is fine. The goitrogenic-vegetable concern (cabbage, broccoli, kale) is overstated — it’s only meaningful in the setting of frank iodine deficiency and very large daily raw intake. Cruciferous vegetables in normal dietary amounts are protective, not harmful.

Alcohol. Doesn’t directly interact, but heavy drinking affects sleep, gut function, and the broader picture in ways that don’t help thyroid stability.

Generic substitution at the pharmacy. As above — bioequivalent on paper, occasionally not in felt experience. Once your dose is settled, ask to stay with the same brand and tell the pharmacist.

• TSH 6-8 weeks after starting or after any dose change. Earlier than this gives an unreliable picture because steady state hasn’t been reached.
• TSH + free T4 at the same point if there’s any concern about pituitary function or unusual symptoms.
• TSH annually once stable, ideally with your routine review. More often if pregnancy, weight changes, new oestrogen-containing medicine, new PPI, or new symptoms.
• TFTs every 4 weeks through pregnancy, with trimester-specific targets (T1 <2.5, T2 <3.0, T3 <3.0 mIU/L per the ATA 2017 pregnancy guideline).
• Thyroid antibodies (TPO, TG) — checked at diagnosis to establish whether the cause is autoimmune (Hashimoto’s). Trending antibodies doesn’t change management for most people; doing it once is usually enough.
• A broader nutrient panel is reasonable at diagnosis and when symptoms aren’t matching the TSH — iron studies (with ferritin), vitamin B12, vitamin D, sometimes selenium and zinc.

If your TSH won’t settle despite dose increases, the issue is usually not the dose. Absorption (coffee timing, calcium timing, PPI use, gut conditions like coeliac), adherence, or a recent brand change are the first places to look.

This is the section many patients come looking for, so it deserves a careful answer rather than a quick dismissal.

Where mainstream guidance currently sits. eTG, NICE NG145, and the American Thyroid Association 2014 guideline all place levothyroxine monotherapy as first-line for almost everyone with primary hypothyroidism. The ATA 2021 consensus statement acknowledges that a minority of patients remain dissatisfied on levothyroxine alone despite biochemically normalised TFTs, and that a carefully-monitored trial of T4+T3 combination is reasonable for that group — typically with specialist input. NDT is not endorsed as first-line in any of the major guidelines, but the same body of work acknowledges patient preference is real and meaningful.

Where the patient-experience evidence sits. Bianco and Kim’s 2019 JCI paper reviews the physiological and genetic plausibility for dissatisfaction with levothyroxine — including DIO2 polymorphism, which alters T4-to-T3 conversion efficiency in a subset of people. The Hoang 2013 JCEM crossover trial of NDT versus levothyroxine showed no consistent biochemical advantage but a patient-preference shift toward NDT in a meaningful minority. Hold both of these things in your head at the same time: the trial evidence is mixed and short in duration; and patient preference for NDT in a subset is consistent and not adequately explained by placebo alone.

Where the AU access reality sits. No TGA-registered NDT brand is consistently available in Australia at the time of writing. Access is via:

• A compounding pharmacy that prepares NDT to prescription, or
• The special-access scheme for an overseas-sourced product.

Both pathways involve cost, prescription continuity, and sometimes supply gaps. Cost is meaningfully higher than PBS-listed levothyroxine.

Where T4+T3 combination sits. Tertroxin (synthetic T3) has historically had intermittent AU supply — confirm with the pharmacist before relying on it. When it’s available, a T4+T3 combination is usually trialled with endocrinology input, with the T4 dose reduced and T3 added at small doses (typically 5-10 mcg, divided through the day). The aim is to mimic the body’s roughly 14:1 production ratio, which is closer to physiological than NDT’s 4:1 ratio.

How the conversation usually goes. Most patients who arrive asking about NDT or T4+T3 have one of three pictures:

1. Persistent symptoms despite a TSH in the lower-normal range, on a settled levothyroxine dose. Worth working through nutrient co-factors (selenium, iron, B12, vitamin D), absorption (coffee timing, PPI use), sleep, perimenopause, and broader life-load before a T3 trial. Often the lever is upstream.
2. Persistent symptoms despite all of the above being addressed, on a clearly adequate levothyroxine dose for at least 6 months. This is the group where a T4+T3 trial — or, less commonly, NDT — has a real chance of helping, ideally in shared care with endocrinology.
3. A strong prior preference for “natural” treatment based on reading and lived experience. This is a reasonable starting point for a conversation; it’s not a reason to switch in isolation. Worth understanding the underlying physiology and the actual trial evidence before committing to a change.

The choice is yours. Each path has a different consequence — biochemical, financial, and in some cases insurance/work-cover (specialty drug formularies). The point of the consult is to walk through which picture you’re in and what fits.

Pregnancy raises requirement.

Alexander et al. (NEJM 2004) showed that levothyroxine requirement rises by roughly 25-30% from early in the first trimester, with most of the increase needed by 4-6 weeks gestation. The subsequent ATA 2017 pregnancy guideline codified the trimester-specific TSH targets used in AU practice. The mechanism: oestrogen raises thyroxine-binding globulin (shifting hormone into the inactive bound pool), and the baby draws on maternal thyroid hormone supply before the baby’s own thyroid is functional. The increase is needed early — usually within the first 4-6 weeks of pregnancy.

Practical approach when pregnancy is confirmed:

• Notify your GP as soon as you have a positive pregnancy test. Don’t wait for the booking visit.
• A common move is to increase the weekly dose by two extra tablets straight away (i.e. 9 doses per week instead of 7), then re-check TSH at 4 weeks. Your GP or endocrinologist will tailor this to your starting dose.
• TSH every 4 weeks through pregnancy, with trimester-specific targets — the ATA 2017 guideline suggests T1 <2.5, T2 <3.0, T3 <3.0 mIU/L.
• After delivery, dose usually returns to pre-pregnancy levels. A 6-8 week post-partum TSH confirms the right post-baby dose.

Planning pregnancy. If you’re trying to conceive, it’s worth confirming TSH is in the lower-normal range (ideally <2.5 mIU/L) before pregnancy. This is the calmest version of starting the increase.

Breastfeeding. Levothyroxine is safe in breastfeeding — only trivial amounts cross into milk. Liothyronine and NDT are less well-characterised in lactation; specialist input is reasonable if you’re on those.

Thyroxine is not a weight-loss medicine in people with a normally functioning thyroid. Requests for thyroxine to lose weight when TFTs are normal will not be prescribed. Using thyroid hormone above your physiological replacement dose drives the body into a thyrotoxic state — it accelerates bone loss, strains the heart, raises atrial fibrillation risk (particularly in adults over 65), and the weight-loss effect is small, not durable, and not safe.

It’s not an energy supplement for people whose thyroid is working. The “I just want a small boost” framing reflects an understandable wish but isn’t the right tool. If energy is the issue and TFTs are normal, the lever is upstream — sleep architecture, iron and ferritin status, B12, vitamin D, cortisol pattern, perimenopause, autoimmune screen, blood glucose stability, and broader life-load all produce the same felt picture.

It’s not for self-titration outside TFT monitoring. Small dose changes produce large TSH swings; the consequences of sustained over-replacement — atrial fibrillation, bone loss, cardiovascular strain — accrue silently. The monitoring is what keeps the medicine safe.

Don’t stop without talking to us first.

• If side effects are the problem, the issue is usually dose or brand, not the molecule. Adjusting the dose or returning to a previous brand resolves it for most people.
• Severe gastro — a missed dose or two won’t shift TSH meaningfully. Resume normal dosing when you can.
• Before surgery — usually fine to take it normally or skip the morning dose and take it after. Anaesthetic teams are familiar with this.
• Stopping for “natural” treatment — for established primary hypothyroidism (after thyroidectomy, post-radioactive iodine, or longstanding Hashimoto’s with elevated TSH), the gland is not making enough hormone and won’t begin to. Stopping leads to a return of hypothyroid symptoms over weeks to months. For subclinical hypothyroidism (TSH 5-10, antibodies positive, mild symptoms), there’s a small subset of people in whom dose can come down or off after sustained nutrient repletion and autoimmune-modulating work — under monitoring.

Replacement after total or near-total thyroidectomy, or after definitive radioactive iodine for Graves’ disease or thyroid cancer, is structurally different from replacement for Hashimoto’s:

• The gland is not coming back. The dose is permanent; the question is what’s the right dose.
• Doses are usually higher than for Hashimoto’s — typically 1.5-1.7 mcg per kg body weight per day for full replacement, occasionally higher for TSH-suppressed dosing in thyroid cancer follow-up.
• TSH suppression for differentiated thyroid cancer — endocrinology sets the target. The aim is usually a TSH in the lower-normal range or actively suppressed depending on risk stratification. Long-term suppressed TSH carries bone density and cardiac considerations that get factored into the dose.

If you’re in this group, the endocrinology team usually leads dose decisions and we share the long-term monitoring.

Both levothyroxine and liothyronine are PBS-listed unrestricted. From 1 January 2026, the PBS co-payment is:

• General patient: up to $25.00 per script
• Concession card holder: up to $7.70 per script

Levothyroxine is commonly priced under the co-payment threshold, so your actual charge may be lower — ask the pharmacist for the under-co-payment price. Liothyronine (Tertroxin) tends to be at the co-payment level.

NDT is not PBS-listed. Compounded NDT or special-access NDT is priced privately by the compounding pharmacy or supplier, typically several times the levothyroxine cost per month. Continuity of supply can be an issue, particularly for special-access products.

Generic levothyroxine costs the same as branded at PBS pricing and works the same — but the brand-consistency caveat above applies: once your dose is settled, stay with one brand and tell the pharmacist if a substitution is offered.