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Thyroid and weight: the arrow points the other way
Research review · PubMed
✍️ Автор: Катя
Hypothyroidism is a real illness and we are not going to trivialise it. But a mythology has grown around it — twenty kilograms, a broken metabolism, gluten-free protocols. We opened the Institute of Medicine reference intakes, the American Thyroid Association position, Cochrane reviews and genetic studies, and got three numbers that change the whole picture.
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How many kilograms are genuinely «the thyroid»
The official American Thyroid Association position: **5 to 10 pounds**, roughly 2.3–4.5 kg. And immediately, the crucial qualifier from the same document: «**most of the weight gained is actually due to retaining salt and water**». Not fat.
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And direct measurement confirmed it
Patients with severe hypothyroidism were treated and weighed a year later: **minus 4.3 kg**. But when researchers looked at *what* had gone, **fat mass had not changed at all**. The authors' verbatim conclusion: the weight loss is «caused by **excretion of excess body water**». Two further studies replicated it: fat mass unchanged.
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How far metabolism actually drops
Also measured precisely: from 1330 to 1442 kcal per day after treatment. So hypothyroidism costs you **113 kcal a day, or 8.5%**. That is one banana. A real, measurable deficit — but not the one that explains twenty kilograms. And it disappears entirely on treatment.
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Now the main point: the arrow runs the other way
A Mendelian randomisation study tested both directions. The result: genetically higher TSH **does not cause obesity**. But genetically higher body mass index **does raise TSH**, significantly. In other words the thyroid is not making people heavier — **higher weight is raising TSH**.
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And this is confirmed the best way possible
A meta-analysis of 28 studies in 1,284 patients after bariatric surgery: TSH fell by **1.66**, and subclinical hypothyroidism **resolved completely in 87%** of people. The authors of one study put it directly: it was «just a **consequence of abnormal fat accumulation**, not a real hypothyroid state». The good news follows: if a mildly raised TSH is a consequence, it resolves along with the weight, and no lifelong tablet is needed.
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Another driver of over-diagnosis: age
TSH reference ranges depend on age, and heavily. The upper limit is **3.56** at 20–29 and **7.49** in people over 80. The verbatim finding: «**70% of older patients with TSH greater than 4.5 were within their age-specific reference range**». What looks like disease on a lab report is often just being 75.
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Subclinical hypothyroidism: treat it to lose weight?
The best trial available — 737 people, levothyroxine versus placebo. TSH fell nicely. The change in how people felt: **0.2 in one group and 0.2 in the other**. Zero difference. A meta-analysis across 2,192 participants confirmed it: neither quality of life nor symptoms improve. On weight in obesity, the American Thyroid Association writes: «**no recommendation due to insufficient evidence**».
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And why «a little thyroid hormone will not hurt» is a dangerous idea
Overtreatment has a concrete price. With suppressed TSH the risk of atrial fibrillation **triples** — 28% versus 11% over ten years. And a meta-analysis of 70,000 people found that at the lowest TSH the risk of vertebral fracture is **3.6 times higher**. An important fairness note: undertreatment harms about equally. The message is not «fear the hormone» but **keep TSH in range**.
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The loudest finding: what is actually in «thyroid support» supplements
Researchers bought **ten commercial supplements** and analysed them. **Nine of ten contained genuine T3 hormone.** Five contained T4. At the manufacturers' recommended dosing, four delivered up to **91.6 mcg of T4 per day** — a full therapeutic dose, taken blind, without blood tests, from a jar labelled «natural support».
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Selenium: the number moves, health does not
Selenium does lower antibodies. But **gland volume does not change, thyroglobulin antibodies do not change**, and nobody has demonstrated any effect on disease progression, symptoms or quality of life. Cochrane's verdict: the evidence is «incomplete and not reliable» for clinical decisions. The harm, meanwhile, is concrete: in a trial of 1,202 people, **200 mcg per day** — the exact dose in «Hashimoto protocols» — raised the risk of type 2 diabetes **by 55%**.
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Iodine: both deficiency and excess do damage
A prospective Chinese study: with excess iodine, subclinical hypothyroidism occurred in **2.9%** versus **0.2%** at normal intake, and autoimmune thyroiditis in 1.3% versus 0.2%. Now the practical arithmetic. A sheet of nori (~3 g) is **48 mcg of iodine** — a third of the daily requirement, entirely safe. But **one teaspoon of kelp granules is 16,330 mcg**, fifteen times the maximum tolerable intake. That is the difference between sushi and a «natural» supplement.
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«Do not eat cabbage and broccoli» — let us count
The threshold at which goitrogens start blocking iodine uptake is 194 units. Broccoli, cabbage and turnip greens contain **fewer than 10 per 100 g serving**. To reach the threshold you would need to eat roughly **2 kg of raw broccoli in one sitting**. Cases of harm do exist in the literature, but they involve a woman who ate large amounts of raw bok choy every day for months.
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How to take levothyroxine properly
The difference is large: TSH of **1.06 fasting versus 2.93 with breakfast**. The recommendation is **60 minutes before breakfast, or at bedtime** (3+ hours after your evening meal). What blocks absorption: coffee (minus 27–36%), calcium (TSH from 1.6 to 2.7), iron (TSH from 1.6 to 5.4 over 12 weeks). All of it is solved by **separating them in time**, not by giving them up.
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The bottom line
Hypothyroidism is real — but the overt form affects **0.3% of people**, costs 113 kcal a day and holds 3–4 kg of **water**. Testing your TSH because you cannot lose weight is sensible and cheap. Just be ready for it to come back normal, in which case the answer lies elsewhere. And remember the American Thyroid Association's own rule: **if treatment cleared the fatigue and the cold intolerance but the weight has not moved, the weight was never the thyroid**.
❓ Часті запитання
How much weight can hypothyroidism account for?
The American Thyroid Association puts it at 5–10 pounds, most of which is retained salt and water. After treatment people lose about 4 kg, but measurement shows fat mass does not change.
Will treating my thyroid make me lose weight?
What is lost is mostly water, not fat. Once hormone levels normalise, your ability to lose weight is the same as in people without thyroid disease.
Should subclinical hypothyroidism be treated for weight?
A trial in 737 people found no difference in how patients felt between levothyroxine and placebo. On weight in obesity the official position is that there is no recommendation, due to insufficient evidence.
Is selenium useful in Hashimoto's?
It lowers antibodies, but no clinical improvement has been shown and Cochrane calls the evidence unreliable. Meanwhile 200 mcg per day raised type 2 diabetes risk by 55% in a long-term trial.
Should I avoid cabbage and broccoli?
No. Reaching the threshold would require roughly 2 kg of raw broccoli in one sitting. At ordinary portions there is no risk.
Are «thyroid support» supplements safe?
In one analysis, 9 of 10 such products contained genuine T3 hormone, and some delivered up to 91.6 mcg of T4 per day — a therapeutic dose with no blood monitoring.
📚 Дослідження, згадані в статті (16)
- KEY: weight lost after treatment is water, not fat (JCEM, 2011) — джерело →
- Mendelian randomisation: obesity raises TSH, not the reverse (JCEM, 2021) — джерело →
- After weight loss, subclinical hypothyroidism resolves in 87%, 28 studies (Surg Obes Relat Dis, 2021) — джерело →
- Metabolic rate in hypothyroidism: −113 kcal/day (Thyroid, 2019) — джерело →
- TRUST: levothyroxine did not improve symptoms, 737 people (NEJM, 2017) — джерело →
- Meta-analysis of all RCTs: quality of life unchanged, 2,192 participants (JAMA, 2018) — джерело →
- Age-specific TSH ranges: 3.56 in the young vs 7.49 at 80+ (JCEM, 2007) — джерело →
- 9 of 10 «thyroid support» supplements contained T3 (Thyroid, 2013) — джерело →
- Selenium 200 mcg/day raised diabetes risk by 55%, 1,202 people (Ann Intern Med, 2007) — джерело →
- Selenium in Hashimoto's: no clinical improvement shown (Thyroid, 2024) — джерело →
- Excess iodine causes hypothyroidism and thyroiditis (NEJM, 2006) — джерело →
- Iodine content of seaweeds: kelp 8,165 mcg/g (Thyroid, 2004) — джерело →
- Goitrogens: threshold 194 units, broccoli under 10 (Nutrition Reviews, 2016) — джерело →
- Fasting vs with breakfast: TSH 1.06 vs 2.93 (JCEM, 2009) — джерело →
- Suppressed TSH and atrial fibrillation: risk tripled (NEJM, 1994) — джерело →
- Overtreatment and fractures: 70,298 participants (JAMA, 2015) — джерело →
Це не медична порада. Маєш діагноз чи обмеження — звернися до лікаря.
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