我一定要間隔 4 小時嗎？3 小時可以嗎？

文獻上「4 小時」是安全下限。Levothyroxine 血中濃度約在 2–4 小時到達高峰，4 小時後腸腔內剩下的量已很少，鈣結合造成的吸收損失可忽略。3 小時仍在吸收窗口內，仍會造成 10–20% 的吸收下降。若排程真的沒辦法，把 levothyroxine 改成睡前服用、鈣留在白天，是最容易達成 4 小時分離的方案。

我吃的是檸檬酸鈣，不是碳酸鈣，這樣有差嗎？

沒有差。Zamfirescu 2011（Thyroid）直接比較碳酸鈣、檸檬酸鈣、醋酸鈣同時與 levothyroxine 併服，三者都造成約 20–25% 的吸收下降。造成結合的是鈣離子本身，不是鹽類形式。換鹽類不會解決問題，錯開時間才會。

早餐一杯牛奶加無糖優格算嗎？

算，但影響比補充錠小。一杯 240 ml 牛奶約含 300 mg 鈣，仍會在腸腔和 levothyroxine 結合。這也是為什麼標準指示是「空腹配水、30–60 分鐘內不要吃早餐」。若你習慣早餐一定要有牛奶，比較穩定的做法是把 levothyroxine 改成睡前服用。

停經後醫師要我補鈣＋維生素 D 保骨，同時我又在吃 levothyroxine，我該怎麼安排？

這是門診最常見的組合。標準排法是：早上空腹服 levothyroxine 加開水；鈣＋維生素 D 分到午餐、晚餐、睡前吃（分次吸收也比較好）。任何一顆鈣、任何一顆綜合維他命含鈣、任何一顆胃藥含鋁鎂鈣，都要離 levothyroxine 至少 4 小時。啟動或停止補鈣後 6–8 週，請回診複查 TSH，讓醫師評估是否需要調整處方藥劑量——這件事不由你或 App 決定。

我最近一直覺得疲倦、變胖、怕冷，會不會就是鈣讓 levothyroxine 沒吸收到？

有可能，但別自行加劑量。這些症狀符合甲狀腺功能低下再度出現，鈣共服是常見原因之一，其他還有：早餐配咖啡、開始吃鐵劑或胃藥（PPI）、換學名藥廠牌、腸胃手術後吸收改變。正確做法是：立刻把所有含鈣、鐵、鋁的補充品和 levothyroxine 錯開 4 小時，並回診驗 TSH。是否要調整處方藥劑量，一律由醫師決定。

Calcium and Levothyroxine: Why Timing Is Everything

# Calcium and Levothyroxine: Timing Is Everything

> Quick answer. Calcium supplements — carbonate, citrate, phosphate — bind levothyroxine (Synthroid, Euthyrox, Eltroxin) in the gut and cut its absorption by 20–30% on average, occasionally more. You almost never have to stop calcium. You just have to separate them by at least 4 hours. Take levothyroxine on an empty stomach in the morning; take calcium with lunch, dinner, or bedtime.

Levothyroxine has one of the narrowest therapeutic windows in outpatient prescribing. A 20% drop in absorption can move a patient from well-controlled hypothyroidism into a symptomatic, TSH-elevated state — fatigue, weight gain, cold intolerance, high cholesterol — without the patient (or sometimes the clinician) realising that a bedtime calcium supplement is the reason.

This is not a "may interact" footnote. Calcium — along with iron, magnesium, aluminium-containing antacids, sucralfate, PPIs, and high-fibre/soy meals — is one of the most consistently documented absorption reducers of oral thyroid hormone in the literature. It is also one of the easiest to fix.

Mechanism: chelation, not metabolism

Calcium does not touch how your liver processes thyroid hormone. It does not affect deiodinase enzymes or T4-to-T3 conversion. The problem is purely intra-lumen: in the small intestine, calcium cations (Ca²⁺) form an insoluble complex with levothyroxine molecules, and the complex passes through the gut without being absorbed.

Three practical consequences follow from the mechanism:

Time-separation works. Once levothyroxine has crossed the intestinal wall (roughly 3–4 hours after ingestion), calcium in the lumen can no longer capture it. This is why the standard advice is a 4-hour gap, not "avoid calcium".
All calcium salts do it. Carbonate, citrate, phosphate, hydroxyapatite — the cation is the problem, not the counter-ion. Do not switch salt forms hoping to solve the interaction.
Dietary calcium matters, but less. A glass of milk with breakfast is a smaller effect than a 500 mg calcium carbonate tablet, but it still reduces absorption. This is one reason "empty stomach, water only" is the classical instruction.

What the trials actually show

Singh 2000 (Thyroid, 20 hypothyroid adults, crossover). Adding 1,200 mg elemental calcium carbonate with levothyroxine for 3 months raised mean TSH from 1.6 to 2.7 mIU/L. Free T4 fell. Effect reversed on washout. This is the canonical trial cited by NIH ODS.
Zamfirescu 2011 (Thyroid, prospective crossover). Compared calcium carbonate, calcium citrate, and calcium acetate co-ingested with levothyroxine. All three reduced levothyroxine AUC by a similar 20–25%. No advantage to citrate.
Csako 2001 (Journal of Clinical Pharmacy and Therapeutics, case series). Six patients on stable levothyroxine developed elevated TSH after starting calcium carbonate. Separation of dosing restored TSH to previous levels without changing the levothyroxine dose.
Diskin 2007 (Endocrine Practice, dialysis patients). Phosphate binders — including calcium acetate and calcium carbonate — significantly increased levothyroxine requirements. Relevant to CKD/dialysis patients where binder timing is often locked to meals.

The pattern across studies is remarkably stable: ~20–30% reduction in absorption, reversible with time-separation, no dose form of calcium is safer than another.

The 4-hour rule, in practice

Levothyroxine peaks in plasma at 2–4 hours. By the 4-hour mark, the majority has left the intestinal lumen and entered systemic circulation. This is why every major endocrine society converges on the same interval:

American Thyroid Association (ATA 2014 hypothyroidism guidelines). "Concomitant use of calcium carbonate, ferrous sulfate, or aluminum hydroxide can decrease levothyroxine absorption. These should be separated by at least 4 hours."
Endocrine Society. Same 4-hour separation, plus reminder that PPIs, coffee within 60 minutes, and high-fibre meals independently reduce absorption.
UK NICE CKS. Recommends a minimum 4-hour gap from calcium salts, iron salts, and antacids.

Two practical schedules that work for most people:

Schedule A — morning thyroxine, evening calcium.

06:30 Levothyroxine + water, empty stomach.
07:00 (or later) Coffee, breakfast, other supplements.
12:00 / 18:00 / 21:00 Calcium doses with meals.

Schedule B — bedtime thyroxine.

07:00 Breakfast + calcium (if that is when you naturally take it).
12:00 Lunch.
22:30 Levothyroxine, at least 3 hours after the last calcium-containing meal or supplement, before bed.

Bedtime dosing is supported by Bolk 2010 (Archives of Internal Medicine RCT) as equally effective or slightly superior to morning dosing in achieving TSH targets, provided the same food/supplement separation is respected.

Who is at highest risk

Absorption issues become clinically loud in these groups:

Post-menopausal women on both levothyroxine and calcium+vitamin D for bone health. This is the single most common overlap in outpatient endocrine practice.
Patients after thyroidectomy or radioiodine ablation who depend 100% on exogenous levothyroxine. Small absorption shifts translate directly to TSH.
Patients with elevated TSH targets (thyroid cancer suppression). Under-absorption defeats the purpose of the suppression regimen.
Older adults on polypharmacy where calcium/vitamin D, iron, PPIs, and levothyroxine are frequently co-prescribed without timing review.
CKD/dialysis patients on calcium- or lanthanum-based phosphate binders taken with every meal.

Safer use

Confirm morning empty-stomach dosing (or consistent bedtime dosing), water only, 30–60 minutes before food or other supplements.
Keep any calcium — supplement or antacid — at least 4 hours away from the levothyroxine dose.
Don't switch calcium salt forms hoping to solve the interaction. Time-separate instead.
Watch for silent additions: calcium-fortified plant milks, TUMS-style antacids, multi-mineral supplements, fortified breakfast cereals all count.
Recheck TSH 6–8 weeks after starting or stopping calcium, or after any significant schedule change.
Never adjust levothyroxine dose yourself to "compensate" for calcium — the fix is separation, not up-titration. Any dose change is the prescribing clinician's call.

How Glowbit handles this

Glowbit tracks levothyroxine in `profiles.current_medications` (context only — Glowbit does not manage prescription doses) and calcium as a supplement under the `steady_state_target` kinetic class. When both are present, Glowbit:

Flags timing conflicts at add-time and at every schedule change, requiring a minimum 4-hour separation.
Suggests moving calcium to lunch/dinner/bedtime slots if it collides with the morning thyroxine window.
Applies the tolerable upper intake level (UL 2,500 mg/day adults, 2,000 mg over 50) against the sum of supplements + significant dietary sources, not just supplements alone.
Uses the standard Glowbit prescription-safety line whenever a supplement interacts with an Rx drug:

> You are currently taking [levothyroxine]. We do not recommend the current calcium schedule, which may reduce its absorption. Please separate the two by at least 4 hours, or ask your physician or pharmacist for personalised adjustment.

Glowbit does not suggest changing, stopping, or adjusting the levothyroxine dose. That is out of scope for a supplement-safety tool.

Sources

NIH Office of Dietary Supplements — Calcium Fact Sheet for Health Professionals (2024).
Singh N. et al. *Effect of calcium carbonate on the absorption of levothyroxine.* Thyroid 2000.
Zamfirescu I., Carlson H. *Absorption of levothyroxine when coadministered with various calcium formulations.* Thyroid 2011.
Bolk N. et al. *Effects of evening vs morning levothyroxine intake: a randomised trial.* Arch Intern Med 2010.
Jonklaas J. et al. *Guidelines for the Treatment of Hypothyroidism.* American Thyroid Association, Thyroid 2014.
NICE Clinical Knowledge Summaries — Hypothyroidism (2024 update).
Diskin C.J. et al. *Effect of phosphate binders on levothyroxine.* Endocr Pract 2007.

常見問題

讀者常問的問題

我一定要間隔 4 小時嗎？3 小時可以嗎？: 文獻上「4 小時」是安全下限。Levothyroxine 血中濃度約在 2–4 小時到達高峰，4 小時後腸腔內剩下的量已很少，鈣結合造成的吸收損失可忽略。3 小時仍在吸收窗口內，仍會造成 10–20% 的吸收下降。若排程真的沒辦法，把 levothyroxine 改成睡前服用、鈣留在白天，是最容易達成 4 小時分離的方案。
我吃的是檸檬酸鈣，不是碳酸鈣，這樣有差嗎？: 沒有差。Zamfirescu 2011（Thyroid）直接比較碳酸鈣、檸檬酸鈣、醋酸鈣同時與 levothyroxine 併服，三者都造成約 20–25% 的吸收下降。造成結合的是鈣離子本身，不是鹽類形式。換鹽類不會解決問題，錯開時間才會。
早餐一杯牛奶加無糖優格算嗎？: 算，但影響比補充錠小。一杯 240 ml 牛奶約含 300 mg 鈣，仍會在腸腔和 levothyroxine 結合。這也是為什麼標準指示是「空腹配水、30–60 分鐘內不要吃早餐」。若你習慣早餐一定要有牛奶，比較穩定的做法是把 levothyroxine 改成睡前服用。
停經後醫師要我補鈣＋維生素 D 保骨，同時我又在吃 levothyroxine，我該怎麼安排？: 這是門診最常見的組合。標準排法是：早上空腹服 levothyroxine 加開水；鈣＋維生素 D 分到午餐、晚餐、睡前吃（分次吸收也比較好）。任何一顆鈣、任何一顆綜合維他命含鈣、任何一顆胃藥含鋁鎂鈣，都要離 levothyroxine 至少 4 小時。啟動或停止補鈣後 6–8 週，請回診複查 TSH，讓醫師評估是否需要調整處方藥劑量——這件事不由你或 App 決定。
我最近一直覺得疲倦、變胖、怕冷，會不會就是鈣讓 levothyroxine 沒吸收到？: 有可能，但別自行加劑量。這些症狀符合甲狀腺功能低下再度出現，鈣共服是常見原因之一，其他還有：早餐配咖啡、開始吃鐵劑或胃藥（PPI）、換學名藥廠牌、腸胃手術後吸收改變。正確做法是：立刻把所有含鈣、鐵、鋁的補充品和 levothyroxine 錯開 4 小時，並回診驗 TSH。是否要調整處方藥劑量，一律由醫師決定。