Levothyroxin: the pathway in the body

Levothyroxin is part of the pathway “Levothyroxin”. This page shows the whole pathway; the station of Levothyroxin is highlighted.

Where this laboratory value sits: Levothyroxine — synthetic T4. Levothyroxine is swallowed as a tablet. It is chemically the same molecule as the T4 that the thyroid gland releases itself; the label calls it a synthetic T4. It enters the same pool as the body's own T4. Source 10, 1

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. Plenty of T4 curbs TSH release.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Levothyroxine → Uptake in the bowel Most of it passes into the blood from the jejunum and the upper ileum. How much arrives varies; how full the stomach and gut are and dietary fibre both play a part. Calcium and iron salts bind part of it in the gut. Source 10, 1↑ supplies Here it is settled how much T4 passes from the gut into the blood. Calcium and iron salts bind levothyroxine in the gut into compounds that do not cross over; dietary fibre also holds back a share. established physiology Source 1, 3
   ⚖ When the balance tips

   too much — If a lot of levothyroxine crosses over, bound and free T4 in the blood rise, and more of it reaches the cells of the tissues.

   too little — If little crosses over, for example because calcium or iron binds the substance in the gut or little stomach acid is present, a share stays in the gut and leaves the body with the stool.

   observed in studies · Source 1, 2

2. 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. Free T4 is slowly replenished from this pool. Source 10, 4↑ supplies Binding to TBG, TBPA and albumin turns the blood into a large T4 pool from which free T4 is slowly replenished. This is why T4 stays in the circulation for a long time. established physiology Source 10, 4
   ⚖ When the balance tips

   too much — If a lot of T4 is present in the blood, it curbs the release of TRH and TSH via feedback to the hypothalamus and pituitary gland.

   too little — If little T4 is present in the blood, this brake is weaker, and the pituitary gland releases more TSH.

   established physiology · Source 9

3. 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. Without carriers the hormone stays outside the cell. Source 4↑ supplies Carriers such as MCT8 let T4 into the cells; without them the hormone stays outside, however much is present in the blood. So the number of carriers also decides what a tissue receives. established physiology Source 4
   ⚖ When the balance tips

   too much — If a lot of free T4 enters the cells, more starting material is available there for conversion; which share becomes T3 is then set by the deiodinases.

   too little — If little free T4 enters, for example because little MCT8 is present, the cell receives little starting material, even if enough T4 is present in the blood.

   established physiology · Source 4

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. The more T3 forms, the more receptors are occupied. Source 4, 5↑ supplies T3 is the form that binds at the receptor; it arises mostly in the tissue itself from T4. The more D1 and D2 work, the more hormone action reaches the cell. established physiology Source 4, 6
   ⚖ When the balance tips

   too much — If a lot of T3 is present, it occupies more receptors in the cell nucleus; D3 then increasingly converts T3 into inactive forms and so limits the action.

   too little — If little T3 is present, for example because the selenium-containing deiodinases have little activity, more receptors stay unoccupied, and the dependent genes are read less often.

   established physiology · Source 4, 5, 6

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. The tissue thus limits the hormone action. Source 4, 5↓ depletes D3 withdraws hormone from the path: reverse T3 does not bind at the receptor, and D3 also breaks down T3 itself. A tissue thus limits the hormone action that reaches it. established physiology Source 4
   ⚖ When the balance tips

   too much — If a lot of reverse T3 forms, more T4 has gone down the inactive path; correspondingly less starting material remains for making T3.

   too little — If little reverse T3 forms, more T4 runs via D1 and D2 to T3, and more hormone action reaches the tissue.

   established physiology · Source 4

6. 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. Without T3 the receptor keeps the genes braked. Source 6↑ supplies The receptor sits at the DNA even without T3 and keeps genes braked there; only bound T3 releases this brake. The number of occupied receptors determines how strong the response is. established physiology Source 6
   ⚖ When the balance tips

   too much — If many receptors are occupied by T3, the matching genes are switched on or off more often; energy turnover and heat production then run faster.

   too little — If few receptors are occupied, braking partner proteins stay bound, and many target genes continue to be read at a reduced rate.

   established physiology · Source 6

7. 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. T3 thus helps set energy turnover and heat production. Source 6↑ supplies Via the genes that are read, T3 provides proteins that help set energy turnover, heat production and heartbeat. Other genes, such as the one for TSH in the pituitary gland, are switched off. established physiology Source 6, 9
   ⚖ When the balance tips

   too much — If the T3-dependent genes are read strongly, metabolism, heat production and heartbeat run faster, and energy stores are used up more quickly.

   too little — If they are read weakly, energy turnover and heat production run more slowly, and the heartbeat is calmer.

   established physiology · Source 6

8. 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. The gland thus concentrates iodide far above the blood level. Source 8, 5↑ supplies NIS concentrates iodide in the thyroid gland far above the level in the blood, driven by the sodium gradient. Without this step the raw material for the gland's own T4 is missing. established physiology Source 8
   ⚖ When the balance tips

   too much — If a great deal of iodide arrives, the thyroid gland temporarily slows its incorporation into thyroglobulin and the number of NIS carriers; it thus limits its own hormone formation.

   too little — If little iodide arrives, the cell makes more NIS under TSH and draws the available iodide more thoroughly from the blood; at the same time proportionally more T3 than T4 forms.

   established physiology · Source 8, 5

9. 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. Only thus does the hormone store in the follicle arise. Source 7, 5↑ supplies Thyroid peroxidase oxidises iodide with hydrogen peroxide and attaches it to tyrosine building blocks; only thus does the store arise from which the gland makes its hormone. Haem iron in the enzyme is needed for this. established physiology Source 7, 5
   ⚖ When the balance tips

   too much — If a lot of iodine is bound to thyroglobulin, there is a large store in the follicles from which the gland can release hormone over a longer time.

   too little — If little iodine is attached, for example because little haem iron is available for thyroid peroxidase, fewer iodinated building blocks form and so less T4.

   established physiology · Source 7, 5

10. 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. In the blood they mix with supplied T4. Source 7, 9↑ supplies The gland releases mostly T4 and less T3. In the blood this hormone mixes with supplied levothyroxine into a shared pool that gives the same feedback to the pituitary gland. established physiology Source 7, 9, 10
    ⚖ When the balance tips

    too much — If the gland releases a lot of T4 and T3, their level in the blood rises, and the release of TRH and TSH is curbed.

    too little — If the gland releases little, for example because T4 from outside curbs TSH release, the T4 coming from outside makes up a larger part of the shared pool.

    established physiology · Source 9, 10

Further stations

• Levothyroxine — synthetic T4 Levothyroxine is swallowed as a tablet. It is chemically the same molecule as the T4 that the thyroid gland releases itself; the label calls it a synthetic T4. It enters the same pool as the body's own T4. Source 10, 1↑ supplies Levothyroxine supplies T4 that does not differ from the body's own T4. It enters the same pool in the blood and then follows the same path up to its conversion into T3. established physiology Source 10
  ⚖ When the balance tips

  too much — If a lot of T4 is present, the pituitary gland releases less TSH and the thyroid gland makes less of its own hormone; at the same time more T3 forms in the tissues.

  too little — If little T4 arrives, the pituitary gland releases more TSH and so drives the thyroid gland's own hormone formation.

  established physiology · Source 9, 4

• T4 in the target cell — starting point of conversion Inside the cell, T4 sits ready as the starting material. Two paths lead away from here; which one prevails is set by each tissue for itself. The balance of deiodinases sets how much action results. Source 4↑ supplies Here T4 is the starting material for two paths: D1 and D2 make T3 from it, D3 makes reverse T3. The balance of these enzymes in the tissue determines how much hormone action results. established physiology Source 4, 5
  ⚖ When the balance tips

  too much — If a lot of T4 is present in the cell, D2 is broken down more quickly; the share that becomes T3 falls, and more runs off via D3.

  too little — If little T4 is present in the cell, D2 lasts longer, and the cell gains more T3 from the scarce starting material.

  established physiology · Source 4

• TSH — from the pituitary gland The pituitary gland releases TSH into the blood. TSH reaches the thyroid gland and there drives every single step of the gland's own hormone formation. Plenty of T4 in the blood curbs its release. Source 9↑ supplies TSH drives the thyroid gland: it stimulates the uptake of iodide, the formation of thyroglobulin and the release of T4 and T3. Free T4 and T3 in the blood curb its release. established physiology Source 9
  ⚖ When the balance tips

  too much — If a lot of TSH is present, the thyroid gland draws in more iodide and makes more hormone; with a lasting stimulus its cell mass also grows.

  too little — If little TSH is present, for example because a lot of T4 in the blood curbs its release, the gland's own hormone formation largely rests, and it releases less T4 and T3.

  established physiology · Source 9

What this path hangs on

• Selenium — The deiodinases that convert T4 into T3 carry selenium as selenocysteine in their active site; scarce selenium slows it Source 4, 5
• Iodine — Every molecule of T4 carries four iodine atoms; on conversion to T3 one of them is freed as iodide; the iodide is reused 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; T4 and T3 arise from it Source 7
• Sodium — Its gradient drives the carrier NIS, which brings iodide into the thyroid cell; without it no iodide enters 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.

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

1. Skelin M, Lucijanić T et al. Factors Affecting Gastrointestinal Absorption of Levothyroxine: A Review. Clin Ther 2017 · PubMed 28153426
2. Liwanpo L, Hershman JM. Conditions and drugs interfering with thyroxine absorption. Best Pract Res Clin Endocrinol Metab 2009 · PubMed 19942153
3. Zamfirescu I, Carlson HE. Absorption of levothyroxine when coadministered with various calcium formulations. Thyroid 2011 · PubMed 21595516
4. Bianco AC, Dumitrescu A et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocr Rev 2019 · PubMed 31033998
5. Köhrle J. Selenium, Iodine and Iron-Essential Trace Elements for Thyroid Hormone Synthesis and Metabolism. Int J Mol Sci 2023 · PubMed 36834802
6. Brent GA. Mechanisms of thyroid hormone action. J Clin Invest 2012 · PubMed 22945636
7. Citterio CE, Targovnik HM, Arvan P. The role of thyroglobulin in thyroid hormonogenesis. Nat Rev Endocrinol 2019 · PubMed 30886364
8. Portulano C, Paroder-Belenitsky M, Carrasco N. The Na+/I- symporter (NIS): mechanism and medical impact. Endocr Rev 2014 · PubMed 24311738
9. Ortiga-Carvalho TM, Chiamolera MI et al. Hypothalamus-Pituitary-Thyroid Axis. Compr Physiol 2016 · PubMed 27347897
10. 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
11. US prescribing information (United States): Levothyroxine Sodium Tablets, DailyMed, version of 17 Sept 2026, section 6 Adverse Reactions · Prescribing information

Whole pathway: Levothyroxin

As of 2026-09-25. Draft, written by Claude to schema v2; sources checked in PubMed; expert approval pending
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