TRH (Protirelin)

A hypothalamic tripeptide (pGlu-His-Pro-NH2) that triggers pituitary TSH and prolactin release; its two U.S. diagnostic injections are both discontinued, and its neuropsychiatric research record is a long series of rapid but transient effects.

Also referenced as: Protirelin, Thyrotropin-Releasing Hormone, TRH, Thyroliberin, Protirelin tartrate, Thyrel TRH, Thypinone, pGlu-His-Pro-NH2, Thyrotropin-releasing factor

Also appears in: Cognitive

What is TRH (Protirelin)?

TRH is a three-residue peptide — pyroglutamyl-histidyl-proline amide, written pGlu-His-Pro-NH2, molecular formula C16H22N6O4 and molecular weight 362.38 — released by neurons of the hypothalamic paraventricular nucleus into the hypophyseal portal circulation, where it drives secretion of thyroid-stimulating hormone from pituitary thyrotropes. It was the first hypothalamic releasing hormone isolated and chemically identified, and its structure was settled in November 1969 by two competing groups within days of each other: the Schally and Bowers laboratory (Boler et al., Biochemical and Biophysical Research Communications, 1969;37:705–710) and the Guillemin laboratory (Burgus et al., Comptes Rendus de l’Académie des Sciences, 1969;269:1870–1873, expanded in Nature, 1970;226:321–325). That work was recognized in the 1977 Nobel Prize in Physiology or Medicine, one half of which went jointly to Roger Guillemin and Andrew Schally for their discoveries concerning peptide hormone production in the brain; the other half went to Rosalyn Yalow for radioimmunoassay. “Protirelin” is the international nonproprietary name for the synthetic tripeptide, chemically identical to the endogenous hormone. Both terminal modifications matter: the N-terminal pyroglutamate and the C-terminal amide are what distinguish TRH from an ordinary tripeptide and are required for receptor activity.

The molecule reached the U.S. market twice, as a diagnostic injection rather than a therapy. Drugs@FDA records Thypinone (Abbott, NDA 017638, a Type 1 new molecular entity approved November 5, 1976) and Thyrel TRH (Ferring, NDA 018087, approved July 18, 1978), both protirelin 0.5 mg/mL for injection, and both now carry a marketing status of Discontinued. Ferring halted production in July 2002 for manufacturing upgrades and no product returned; by then, ultrasensitive TSH assays had already displaced most of the diagnostic rationale. Elsewhere the molecule never left: protirelin and protirelin tartrate hydrate are both listed in the Japanese Pharmacopoeia, and the tartrate (Hirtonin, Takeda) carries a Japanese central-nervous-system therapeutic classification alongside the endocrine-diagnostic one — the one place where TRH itself is used as a treatment rather than a test. What keeps TRH circulating as a research compound in the United States is a separate and much older literature on its central effects — arousal, mood elevation, motor activation — that was never resolved either way, and which reads as a nootropic story when detached from the trials that tested it.

How it works

• TRH-R1 agonism at pituitary thyrotropes and lactotropes. TRH binds thyrotropin-releasing hormone receptor 1, a class A G protein–coupled receptor that signals through Gq/11 to phospholipase C, hydrolyzing PIP2 into inositol trisphosphate and diacylglycerol, mobilizing intracellular calcium and activating protein kinase C — the cascade that triggers TSH and prolactin secretion (Trubacova, Drastichova & Novotny, Frontiers in Cell and Developmental Biology, 2022;10:981452). Circulating thyroid hormone feeds back on TRH gene expression in the paraventricular nucleus, closing the hypothalamic-pituitary-thyroid loop (Chiamolera & Wondisford, Endocrinology, 2009;150:1091–1096).
• Humans have only one TRH receptor. A second subtype, TRH-R2, was cloned from rat and shown to bind TRH and signal acutely much like TRH-R1 while differing in regulation and anatomic distribution, with far wider expression in brain (O’Dowd et al., Molecular Endocrinology, 2000;14:183–193). The two subtypes have indistinguishable binding affinities for TRH but differ in tissue distribution, basal signaling, and internalization (Sun, Lu & Gershengorn, Journal of Molecular Endocrinology, 2003;30:87–97). TRH-R2 is present in rodents and other mammals and absent in humans, who express TRH-R1 only (Trubacova et al., 2022, above; Charli et al., Frontiers in Pharmacology, 2020;11:640). A large share of the rodent CNS pharmacology that supplies the arousal and neuroprotection narrative was generated in animals with a receptor humans do not have, which is a real translation gap rather than a technicality.
• A brisk, self-limiting endocrine signal. Approved protirelin labeling describes a mean plasma half-life of roughly five minutes, with serum TSH rising rapidly to a peak about 20 to 30 minutes after a 500 mcg intravenous dose and approaching baseline again after about three hours. Prolactin rises alongside TSH. The label also notes that approximately 65% of acromegalic patients tested respond with a rise in circulating growth hormone — but states in the same sentence that the clinical significance of that finding is not clear, and acromegaly is not a labeled indication.
• Rapid enzymatic inactivation. TRH is hydrolyzed in blood and extracellular space by pyroglutamyl peptidase II, also called the TRH-degrading ectoenzyme, the one M1-family metallopeptidase with a very narrow specificity — TRH is its best-characterized biological substrate, though two related tripeptide amides are also cleaved (Charli et al., 2020, above). Parallel deamidation converts the peptide to inactive TRH-OH. This is the pharmacokinetic reason exogenous TRH behaves as a pulse rather than a treatment.
• Central effects that do not run through the thyroid axis. TRH neurons and receptors are distributed well beyond the hypothalamus, and pharmacological studies show multiple central effects — arousal, anorexia, anxiolysis, analeptic and anticonvulsant actions, and control of gastric, cardiac and respiratory autonomic function — independent of hypothalamic-pituitary-thyroid control (Charli et al., 2020, above; Fröhlich & Wahl, Frontiers in Neuroendocrinology, 2019;52:29–43). In both human trials that measured mood and thyroid response together, the behavioral effect showed no correlation with the change in TSH, T4, or T3 (Bunevicius & Matulevicius, Psychoneuroendocrinology, 1993;18:445–449; Szuba et al., Journal of Clinical Psychopharmacology, 2005;25:325–330), which is the strongest available evidence that the mood effect, whatever it is, is not a thyroid effect.

Research status

TRH has a large human literature and almost no positive late-stage result. The recurring pattern across five decades is a fast, measurable, short-lived effect in a small sample that does not survive scale-up.

Diagnostic use (the approved indication). The labeled indications were narrow: adjunctive assessment of thyroid function, adjunctive information in patients with pituitary or hypothalamic dysfunction, evaluating whether a given levothyroxine dose adequately suppresses TSH in nodular or diffuse goiter, and adjusting thyroid hormone replacement in primary hypothyroidism. The label is candid about the test’s limits — because a brisk TSH rise occurs in normal subjects too, TRH testing does not differentiate primary hypothyroidism from normal, and a response above 2 µU/mL does not reliably separate pituitary from hypothalamic hypothyroidism. Beyond the label, the TRH stimulation test also had a clinical role in the differential between TSH-secreting pituitary adenoma and resistance to thyroid hormone. That narrow use case survives in countries where protirelin remains marketed. It did not survive commercially in the United States, where sensitive TSH assays answered most of the same questions without an injection.

Depression — the origin of the interest, and a mixed record from the start. Prange and colleagues reported prompt but brief improvement in depressed patients given intravenous TRH (Lancet, 1972;2:999–1002), and Kastin and colleagues published a report in the same journal and year describing improvement in mental depression alongside a decreased thyrotropin response (Lancet, 1972;2:740–742). Replication went badly. A double-blind study of oral TRH at 200–300 mg daily in 11 depressed patients — hospitalized for an initial 10 days, then followed as outpatients over a 30-day period — found not merely absence of benefit but antitherapeutic effects in 3 of the 6 treated patients versus placebo controls, with excessive CNS arousal and psychomotor activation producing dysphoric mental and behavioral effects (Kiely et al., Psychosomatic Medicine, 1976;38:233–241). The largest placebo-controlled study of the intravenous route gave 0.2 mg of protirelin to 26 medication-free depressed women against placebo in 16 others, and found improvement on anxiety and mood scales two hours later that did not correlate with thyroid hormone or TSH changes — effects the authors themselves called short-lasting (Bunevicius & Matulevicius, Psychoneuroendocrinology, 1993;18:445–449). Investigators later attributed the inconsistency to poor blood-brain barrier penetration and changed the route: Marangell and colleagues gave 500 mcg by lumbar intrathecal injection against a sham lumbar puncture in a double-blind crossover of 8 medication-free refractory inpatients, and 5 of 8 met a 50% reduction on an abbreviated Hamilton scale, with responses described as rapid, clinically robust, and short-lived (Archives of General Psychiatry, 1997;54:214–222). A companion report on 2 treatment-refractory bipolar II patients found robust responses by both intravenous and intrathecal routes, with open intravenous dosing effective until apparent tolerance developed (Callahan et al., Biological Psychiatry, 1997;41:264–272). The largest randomized trial in bipolar depression is Szuba et al., who randomized 20 bipolar I or II patients in a depressive episode to intravenous TRH 500 mcg or saline at midnight: 60% of the TRH group versus 10% of the saline group met a 50% Hamilton reduction within 24 hours, with effects lasting up to 48 hours and a modest increase in mania ratings, though no manic or hypomanic episodes occurred (Journal of Clinical Psychopharmacology, 2005;25:325–330). Twenty patients over 48 hours is where this line of work stopped. No Phase 3 antidepressant program for TRH exists.

Amyotrophic lateral sclerosis — a negative controlled trial. Motivated by the high concentration of TRH in spinal anterior horn regions and its excitatory action on motor neurons, TRH was tested in ALS. In a double-blind controlled trial of intramuscular TRH 150 mg daily in 30 patients, given for two months and followed by a two-month washout, a temporary increase in the strength of some muscles was detected — but there was no change in functional performance, neither the patients nor the investigators believed the effects were of any marked clinical significance, and the course of the illness was not altered (Brooke et al., Neurology, 1986;36:146–151). Note the dose: 150 mg daily, three hundred times the 500 mcg used in the diagnostic test, which is the scale required to force a sustained central effect from a peptide with a five-minute half-life.

Spinal cord injury — a strong animal result and a pilot that was never followed up. In cats given cervical spine trauma by the Allen method, six animals per group received TRH, saline, or dexamethasone as a four-hour intravenous infusion beginning one hour after injury; neurologic recovery was significantly better with TRH than with either comparator (p < 0.01), and at six weeks the average TRH-treated animal was normal while average controls had marked spasticity (Faden, Jacobs & Holaday, New England Journal of Medicine, 1981;305:1063–1067). The human follow-on randomized 20 patients with acute injury to TRH or saline. Infusions were well tolerated; there was no discernible effect in complete injuries, and in the incomplete-injury group — 6 treated and 5 placebo patients evaluated at 4 months — TRH was associated with significantly higher motor, sensory, and Sunnybrook scores. The authors explicitly cautioned that the numbers were too small to interpret confidently and called for a larger multicenter trial (Pitts et al., Journal of Neurotrauma, 1995;12:235–243). That trial was never run.

Antenatal lung maturation — the largest controlled program, and it showed harm. The one indication that generated trials at scale was prenatal TRH added to corticosteroids to prevent neonatal respiratory disease. A Cochrane review of 15 randomized trials enrolling over 4,600 women found no reduction in death before hospital discharge (RR 1.05, 95% CI 0.86–1.27), respiratory distress syndrome (average RR 1.05, 95% CI 0.91–1.22), or chronic lung disease (RR 1.01, 95% CI 0.85–1.19), and no improvement in any secondary outcome by intention-to-treat — while every reported maternal side effect was significantly more likely with TRH, and in the infants prenatal TRH increased the risk of needing respiratory support (RR 1.16, 95% CI 1.03–1.29) and of a low five-minute Apgar score (RR 1.48, 95% CI 1.14–1.92) (Crowther et al., Cochrane Database of Systematic Reviews, 2013;CD000019). Twelve-month follow-up of the Australian ACTOBAT cohort found TRH exposure associated with motor delay (OR 1.51, 95% CI 1.11–2.05), social delay (OR 1.40, 95% CI 1.01–1.95), sensory impairment (OR 2.00, 95% CI 1.06–3.74), and severe impairment (OR 1.75, 95% CI 1.07–2.87) after multivariate adjustment (Crowther et al., Pediatrics, 1997;99:311–317). Read that follow-up with the reviewers’ caveat attached: only three of the fifteen trials reported childhood outcomes at all, and while ACTOBAT found these deficits, the two others that used an established developmental instrument showed no clear differences between groups. The practice was abandoned regardless, because there was no benefit to weigh against any of it.

Cancer-related fatigue. A pilot randomized, placebo-controlled crossover study reported significant improvement in fatigue on a visual analog scale for energy and on the POMS fatigue and vigor subscales and the FACIT-F fatigue subscale, with transient increases in blood pressure and heart rate — in 8 completers (Kamath, Feinn & Winokur, Supportive Care in Cancer, 2012;20:1745–1753). The associated Phase 2 record (ClinicalTrials.gov NCT00790296, UConn Health) is listed as terminated with 11 participants enrolled, the registry’s stop reason citing interim analyses showing statistically and clinically significant results; study completion is recorded as March 2010, and no successor trial has been registered since. A small crossover stopped early on a favorable interim is the design most likely to overstate an effect, and it has not been replicated.

Analogs did not rescue the program. Because native TRH is degraded in minutes and penetrates the brain poorly, most sustained development went into longer-acting analogs. Taltirelin is the only TRH analog in general clinical use for a therapeutic — rather than diagnostic — indication, and it is marketed only outside the United States: in Japan since 2000 (Ceredist) and in South Korea (C-Trelin), in both cases for ataxia in spinocerebellar degeneration. It has no U.S. approval and no registered U.S. trial. The largest published randomized test of it is Korean: 149 patients, 5 mg orally twice daily, with a statistically significant but small advantage on the Korean version of the SARA ataxia score at 24 weeks (−0.51 versus 0.36, p = 0.032) and no significant difference on any other secondary measure (Cho et al., Journal of Movement Disorders, 2025;18:35–44, with a 2026 erratum). Rovatirelin (KPS-0373), another TRH derivative, ran a long registered program in spinocerebellar degeneration — Phase 2 studies (NCT00863538, NCT01004016, NCT01384435) followed by confirmatory, long-term, extension and additional-confirmatory Phase 3 studies (NCT01970098, NCT01970111, NCT01970124, NCT01970137, NCT02889302). In the two randomized placebo-controlled Phase 3 trials, KPS1301 (n = 374 randomized) and KPS1305 (n = 203 randomized), the primary endpoint — change in SARA total score — showed no significant difference from placebo in either study; significance appeared only in a pooled analysis restricted to the 278 patients meeting the narrower ataxia entry criteria of the second trial (adjusted mean difference −0.61, 95% CI −1.16 to −0.06, p = 0.029), in a paper co-authored by the sponsor (Nishizawa et al., Journal of Neurology, Neurosurgery and Psychiatry, 2020;91:254–262). The Japanese marketing application filed on December 22, 2021 was withdrawn on July 19, 2023 after the regulator indicated that approval on the existing data would be difficult, and the sponsor announced an additional Phase 3 trial in March 2025.

Registry picture. ClinicalTrials.gov returns 15 studies for protirelin as an intervention (August 2026), and the list is thinner than the count suggests: seven are the rovatirelin spinocerebellar program rather than protirelin itself, four are diagnostic or mechanistic studies of the TRH stimulation test in healthy volunteers and thyroid patients (one of them withdrawn), two are the antenatal respiratory distress trials, one is the terminated cancer-fatigue pilot, and one is a mechanistic study in subclinical hypothyroidism. Broadening the query to “thyrotropin-releasing hormone” returns 29 records, but the additions are largely in vitro fertilization and gonadotropin trials that match on a similar hormone name rather than on TRH — a reminder that registry counts for this compound need to be read study by study. Nothing in the list is an active program testing protirelin for any of the uses that drive research-market interest.

Common dosage forms

• Historical prescription injection. Protirelin 500 mcg in 1 mL, supplied in a single 5 mL vial with methylparaben, propylparaben and sodium chloride, given as an intravenous bolus over 15 to 30 seconds — a presentation sized for one diagnostic procedure rather than for repeat administration. Both U.S. applications for this presentation carry a Discontinued marketing status.
• Compounded injectable solution. A 500 mcg/mL protirelin vial from AnazaoHealth has been listed in DailyMed under the marketing category “unapproved drug other,” carrying the FDA disclaimer that the drug has not been found safe and effective and the labeling has not been approved; that label was last updated in October 2012 and its NDC no longer appears in the FDA’s active National Drug Code directory. This is a compounding-pharmacy product, not a licensed drug.
• Bulk active ingredient. The only current U.S. protirelin entries in the NDC directory are bulk drug substance registrations from active-pharmaceutical-ingredient manufacturers, not finished drug products.
• Research-market lyophilized vials. Powder in sealed glass vials for reconstitution, listed across vendors at a standard 20 mg fill with larger sizes occasionally offered — roughly 40 times the 500 mcg used in the labeled diagnostic procedure and in the intravenous and intrathecal depression studies.
• Routes represented in the literature. Human studies have used intravenous, oral, intranasal, and intrathecal administration; intranasal TRH is a documented TSH stimulus (Szabolcs et al., Acta Endocrinologica, 1989;120:149–154), and intrathecal dosing was introduced specifically to bypass the blood-brain barrier. Research-market supply is overwhelmingly the vial-and-diluent format regardless of which route a given study used.
• Analog tablets are a different product. Oral taltirelin, marketed in Japan and South Korea, is a distinct molecule with its own approval and is not interchangeable with protirelin.

This section describes formats only and is not dosing guidance.

Key considerations

• Verify the regulatory status before repeating it. TRH is frequently described as “FDA-approved” on the strength of its 1970s diagnostic approvals. Both applications — NDA 017638 and NDA 018087 — are listed in Drugs@FDA with a marketing status of Discontinued, and no FDA-approved protirelin product is currently marketed in the United States; the only finished-product entry in DailyMed is an unapproved compounded preparation carrying the standard FDA disclaimer, and current NDC listings are bulk active ingredient. Protirelin does remain marketed elsewhere — in Japan it is a pharmacopoeial drug, and the tartrate salt carries a therapeutic indication there, not merely a diagnostic one. That is a different jurisdiction and a different product, not a U.S. approval.
• Labeled safety is not benign. Approved protirelin labeling reports side effects in about 50% of patients tested and requires that the patient be supine with blood pressure measured before dosing and at frequent intervals through the first 15 minutes, because of transient changes in blood pressure in either direction; more severe hypertension or hypotension with or without syncope was reported in a few patients. The most serious labeled event is pituitary apoplexy requiring acute neurosurgical intervention, reported infrequently in patients with pituitary macroadenomas following acute protirelin administration in the setting of combined anterior pituitary function testing with LHRH and insulin. The label also lists transient amaurosis in patients with pituitary tumors and rare convulsions in patients with predisposing conditions. TRH is a potent prolactin secretagogue, so raising prolactin is an intended pharmacological effect, not an off-target one.
• The evidence pattern is rapid onset, short duration, and no scale-up. The positive human findings are real but small: 8 patients intrathecally, 2 patients across routes, 20 patients over 48 hours, 8 completers in cancer fatigue, 11 evaluable patients in spinal cord injury. Where TRH was tested at scale — 15 antenatal trials in over 4,600 women — the result was no benefit plus measurable harm to mothers and newborns. Tolerance to repeated intravenous dosing was reported in the one study that looked for it. A profile that reads as “fast-acting antidepressant” in isolation reads very differently once the negative and null trials are placed beside it.
• Vial size sits in a gap between two very different literatures. A 20 mg research-market vial holds roughly 40 times the 500 mcg dose used in the labeled diagnostic procedure and in the intravenous and intrathecal depression studies — the studies that produced the positive results. Milligram-scale human protocols do exist, but they are the failures: 150 mg daily intramuscularly in the negative ALS trial, 200–300 mg daily orally in the study that produced dysphoria rather than benefit. So the quantity in a research vial corresponds to no positive human protocol at either end, and a certificate of analysis reporting mass and purity says nothing about whether the amount in the vial matches anything that was ever studied.
• The naming space around TRH is unusually treacherous. TRH (the hypothalamic tripeptide, protirelin) is not TSH (thyrotropin, the pituitary hormone it releases), and recombinant TSH — thyrotropin alfa — is a separate FDA-approved product often confused with it in search results; research listings that title the vial “TRH Thyrotropin” collapse exactly that distinction. Protirelin is not taltirelin or rovatirelin, which are longer-acting synthetic analogs with their own trial records. “TRH test” refers to a diagnostic procedure rather than a therapy. The older designation “TRF,” for thyrotropin-releasing factor, appears throughout the pre-1980 literature. And because rodent TRH-R2 has no human counterpart, animal data indexed under “TRH receptor” may describe a receptor that is not present in the species being marketed to.