Levothyroxin: the pathway in the body
This page shows the biochemical pathway of the active substance Levothyroxin: where it arrives in the body, where it acts and which steps are affected by that. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.
In brief
Levothyroxine is the synthetic form of the thyroid hormone T4 — chemically the same molecule as the body's own. It blocks nothing: it runs the same path, and in tissue deiodinases split off one iodine atom, which yields T3.
What this is about
Levothyroxine is the synthetic T4 — chemically the same molecule the thyroid gland releases itself. It runs the same path in the body, and three points of that path hang on trace elements:
- T4 is not yet the form that acts at the receptor. Deiodinases split off one iodine atom, which yields T3. These enzymes carry selenium as selenocysteine in their active site.
- Every molecule of T4 carries four iodine atoms. On conversion, iodine is freed as iodide and becomes available to the body again.
- The gland's own formation needs two further things: iodide, which the carrier NIS brings into the cell, and thyroid peroxidase — an enzyme with haem iron.
- For uptake in the gut it is described that calcium and iron salts bind the substance when they are present at the same time. Less of it then passes into the blood.
When T4 in the blood rises, the pituitary gland releases less TSH; the thyroid gland then forms less of its own hormone. Supplied and own amounts are thus linked — and the conversion to the active form happens in each tissue for itself, not in the gland. There is no blocking on this path: levothyroxine is the same substance the gland releases itself.
What this means in an individual case depends on many things and belongs in a conversation with a doctor or health practitioner.
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The pathway step by step
- Levothyroxine → Uptake in the bowel Most of it passes into the blood from the jejunum and the upper ileum. How much arrives varies; an empty stomach, dietary fibre and the contents of the gut all play a part. Source 10, 1
- Uptake in the bowel → T4 in the blood In the blood, T4 hangs almost entirely on transport proteins: TBG, TBPA and albumin. Only the unbound share — free T4 — is available to metabolism. Source 10, 4
- T4 in the blood → Free T4 in the cell The unbound share enters the cells via carriers such as MCT8. Only inside the cell is it settled which form the T4 becomes. Source 4
- T4 in the target cell → Free T3 Deiodinase D1, D2 · Selenium The deiodinases D1 and D2 split one iodine atom off the outer ring. This yields T3, the form that binds at the receptor. Both enzymes carry selenium as selenocysteine in their active site. Source 4, 5
- T4 in the target cell → Reverse T3 Deiodinase D3 · Selenium The deiodinase D3 instead removes an iodine atom from the inner ring. The resulting reverse T3 does not fit the receptor and is broken down further. This enzyme too carries selenium. Source 4, 5
- Free T3 → T3 at the receptor In the cell nucleus T3 binds to the thyroid hormone receptor. That receptor already sits at certain stretches of the DNA, mostly together with a second protein. Source 6
- T3 at the receptor → Reading of genes With T3 bound, the receptor changes its shape, lets go of braking partner proteins and draws in reading-promoting ones. The reading of the matching genes is thereby switched on or off. Source 6
- TSH → Iodide in the thyroid NIS · Sodium The carrier NIS brings iodide from the blood into the cell of the thyroid gland. It is driven by sodium, which flows in during the same step. Source 8, 5
- Iodide in the thyroid → Iodine on thyroglobulin TPO · Haem iron, Hydrogen peroxide Thyroid peroxidase attaches iodine to tyrosine building blocks of the large protein thyroglobulin. The enzyme carries haem iron and needs hydrogen peroxide for this. Source 7, 5
- Iodine on thyroglobulin → The gland's own T4, T3 TPO (coupling) The same thyroid peroxidase couples two iodinated building blocks each. This forms T4 and, in smaller amount, T3. The cell frees them from thyroglobulin and releases them into the blood. Source 7, 9
What this path hangs on
- Selenium — The deiodinases that convert T4 into T3 carry selenium as selenocysteine in their active site Source 4, 5
- Iodine — Every molecule of T4 carries four iodine atoms; on conversion to T3 one of them is freed as iodide Source 5, 4
- Iron — Iron salts bind the substance in the gut and lower its uptake; as haem, iron sits in thyroid peroxidase Source 1, 2, 5
- Calcium — Calcium salts bind the substance in the gut; present there at the same time they lower the amount absorbed Source 1, 3
What takes part in these steps
- Tyrosine — Building block in thyroglobulin to which thyroid peroxidase attaches the iodine Source 7
- Sodium — Its gradient drives the carrier NIS, which brings iodide into the thyroid cell Source 8
What acts on this pathway
- Calcium and iron salts — Calcium and iron salts bind levothyroxine in the gut. When they are present at the same time, less of the substance passes into the blood; the label lists calcium carbonate and ferrous sulfate in this group. Source 1, 3, 10
What the prescribing information states
At this point the US prescribing information for the tablets gives no frequency table and no column for a comparator or a placebo. It lists the reactions by organ system and attributes that listing to a dose set too high; below it, under separate headings, it names further reactions, in pediatric patients and against inactive ingredients. A row-by-row comparison is therefore not possible here; the table reproduces what is named there.
How to read the table: unlike with other substances, no figure stands next to any reaction here, and there is no column in which the same report was counted without the substance. The listing therefore shows neither how often something occurs nor how often it would have occurred without the substance.
| Named in the prescribing information | |
|---|---|
| General | Fatigue, increased appetite, weight loss, heat intolerance, fever, heavy sweating |
| Nervous system | Headache, hyperactivity, nervousness, anxiety, irritability, changing mood, insomnia |
| Muscles and skeleton | Tremor, muscle weakness, muscle spasm |
| Heart and circulation | Palpitations, fast heartbeat, rhythm disturbances, rise in pulse and blood pressure, heart failure, angina, myocardial infarction, cardiac arrest |
| Breathing | Shortness of breath |
| Stomach and bowel | Diarrhoea, vomiting, abdominal cramps, altered liver values |
| Skin | Hair loss, flushing, skin rash |
| Endocrine system | Decreased bone mineral density |
| Reproduction | Menstrual irregularities, reduced fertility |
| Seizures | Seizures have been reported rarely with the institution of levothyroxine therapy. |
| In pediatric patients | Pseudotumor cerebri and slipped capital femoral epiphysis have been reported in pediatric patients receiving levothyroxine therapy. Overtreatment may result in craniosynostosis in infants who have not undergone complete closure of the fontanelles, and in premature closure of the epiphyses in pediatric patients still experiencing growth with resultant compromised adult height. |
| Hypersensitivity to inactive ingredients | Hypersensitivity reactions to inactive ingredients have occurred in patients treated with thyroid hormone products. These include urticaria, pruritus, skin rash, flushing, angioedema, various gastrointestinal symptoms (abdominal pain, nausea, vomiting and diarrhea), fever, arthralgia, serum sickness, and wheezing. Hypersensitivity to levothyroxine itself is not known to occur. |
For none of the named reactions does the prescribing information give a frequency; the word “rarely” with the seizures is the only ranking that stands there. The listing by organ system is attributed by the prescribing information to a dose set too high; the reactions in pediatric patients and those against inactive ingredients stand there under separate headings.
Sources
- Skelin M, Lucijanić T et al. Factors Affecting Gastrointestinal Absorption of Levothyroxine: A Review. Clin Ther 2017 · PubMed 28153426
- Liwanpo L, Hershman JM. Conditions and drugs interfering with thyroxine absorption. Best Pract Res Clin Endocrinol Metab 2009 · PubMed 19942153
- Zamfirescu I, Carlson HE. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid 2011 · PubMed 21595516
- Bianco AC, Dumitrescu A et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocr Rev 2019 · PubMed 31033998
- Köhrle J. Selenium, Iodine and Iron-Essential Trace Elements for Thyroid Hormone Synthesis and Metabolism. Int J Mol Sci 2023 · PubMed 36834802
- Brent GA. Mechanisms of thyroid hormone action. J Clin Invest 2012 · PubMed 22945636
- Citterio CE, Targovnik HM, Arvan P. The role of thyroglobulin in thyroid hormonogenesis. Nat Rev Endocrinol 2019 · PubMed 30886364
- Portulano C, Paroder-Belenitsky M, Carrasco N. The Na+/I- symporter (NIS): mechanism and medical impact. Endocr Rev 2014 · PubMed 24311738
- Ortiga-Carvalho TM, Chiamolera MI et al. Hypothalamus-Pituitary-Thyroid Axis. Compr Physiol 2016 · PubMed 27347897
- US prescribing information (United States): Levothyroxine Sodium Tablets, DailyMed, version of 17 Sept 2026, sections 12.1 Mechanism of Action, 12.3 Pharmacokinetics and 7.1 Table 5 · Prescribing information
- US prescribing information (United States): Levothyroxine Sodium Tablets, DailyMed, version of 17 Sept 2026, section 6 Adverse Reactions · Prescribing information
As of 2026-09-25. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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