Follow-up to: when the patient is unable to convert t4 to t3 efficiently?
Question explored with the scientific record
is it not logical that if T3 is supplemented to give the correct levels, there is no reason for the body to raise tsh to produce more t4?
The short version: the evidence does not test your hypothesis. It tests other things entirely, and the gap is the finding.
The retrieved studies are about depression [1, 4, 5], fasting [12], critical illness [11], rexinoid drugs [2], and somatostatin [7]. None of them measure T4-to-T3 conversion efficiency in patients with impaired deiodinase activity. None compare T4-only to T4+T3 therapy. The hypothesis you propose — that exogenous T3 normalizes circulating T3 yet TSH remains driven by local hypothalamic feedback — is plausible on mechanism, but the evidence here does not test it.
The closest signal is the critical illness study [11]: non-survivors had TSH drop to 0.9 µU/mL while T3 fell to 30.6 ng/dL, suggesting the pituitary can suppress TSH even when T3 is low, consistent with central downregulation rather than simple feedback. The fasting review [12] shows that hypothalamic D2 increases during fasting, raising local T3 in the mediobasal hypothalamus while serum T3 falls, which could suppress TRH and TSH independently of circulating T3. That is the mechanism you described, but it is studied in starvation, not in deiodinase deficiency.
| What was studied | Population | Does it test your hypothesis? |
|---|---|---|
| HPT axis in depression [1, 4] | Depressed patients | No — different condition, no conversion measure |
| Fasting and hypothalamic D2 [12] | Rodents and humans | Mechanism only — not deiodinase deficiency |
| Critical illness low T3 [11] | ICU patients | No — different physiology, no T3 intervention |
| Chronic TRH suppresses TSH [13] | Rats | No — tests TRH, not T3 supplementation |
My call: the evidence does not support or refute your hypothesis. The question has not been studied in the right population. Confidence: low, because the gap is large and the mechanistic studies are in different disease contexts.
Sources examined 21
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Triiodothyronine, thyroxine, and tsh response to dexamethasone in depressed patients and normal controls
The study compared TSH, T3, and T4 responses to a 1 mg dexamethasone suppression test in 14 depressed patients (during an episode and after recovery) and 27 healthy controls, finding lower baseline T3/TSH during depression and markedly attenuated TSH suppression by dexamethasone…
DOI: 10.1016/0006-3223(89)90143-1 -
Effects of Rexinoids on Thyrotrope Function and the Hypothalamic-Pituitary-Thyroid Axis
This study investigates the effects of RXR-selective retinoids (rexinoids) on the hypothalamic-pituitary-thyroid (HPT) axis, demonstrating that they directly suppress TSH secretion and TSHβ mRNA levels in thyrotropes, while also affecting deiodinase activity in mice and a thyrot…
DOI: 10.1210/en.2005-0706 -
Long Term Elevations in Plasma Thyrotropin, but not Growth Hormone, Concentrations Associated with Lesion-Induced Depletion of Median Eminence Somatostatin*
The study shows that lesion-induced depletion of somatostatin-containing median eminence (via periventricular hypothalamic PV nucleus lesions) elevates plasma TSH long-term and blocks stress-induced TSH suppression, but does not cause a sustained increase in nonstress GH or bloc…
DOI: 10.1210/endo-112-2-659 -
Mixed anxiety–depressive disorder and major depressive disorder: comparison of the severity of illness and biological variables
This cross-sectional study compared patients with Mixed Anxiety–Depressive Disorder (MADD) and Major Depressive Disorder (MDD) on clinical severity and biological markers (DST, thyroid hormones, basal cortisol), finding higher depressive severity in MDD but higher anxiety in MAD…
DOI: 10.1016/s0165-1781(00)00131-1 -
Hypothalamic‐pituitary‐gonadal axis in major depressive disorders
This study assessed hypothalamic-pituitary-gonadal axis function in major depressive disorder by measuring baseline LH, FSH, and testosterone and the LH/FSH responses to TRH-LHRH, plus DST-based stratification and follow-up remission, finding largely normal gonadotropin levels b…
DOI: 10.1111/j.1600-0447.1988.tb06313.x -
Fatty acid amide hydrolase ablation promotes ectopic lipid storage and insulin resistance due to centrally mediated hypothyroidism
In FAAH knockout mice, elevated endocannabinoid/endocannabinoid-related signaling centrally reduces energy expenditure via activation of hypothalamic PPARγ and suppression of the hypothalamic‑pituitary‑thyroid axis, promoting ectopic hepatic lipid storage and hepatic insulin res…
DOI: 10.1073/pnas.1212887109 -
Somatostatin and the 24 h levels of serum TSH, T3, T4, and reverse T3 in normals, diabetics and patients treated for myxoedema
A 24-hour somatostatin infusion in 5 healthy adults, 7 juvenile diabetics, and 5 patients with treated myxoedema suppresses the nocturnal rise of serum TSH without altering daytime TSH, while causing a notable decrease in T3 (and free T3 in most groups) and a variable effect on …
DOI: 10.1530/acta.0.0940030 -
Inappropriate suppression of thyrotropin during medical treatment of Graves disease in childhood
In a retrospective study of 29 children with Graves disease treated medically, the researchers found episodes of inappropriately low thyrotropin (TSH) despite varying thyroid hormone levels, concluding that TSH is unreliable for diagnosing mild hypothyroidism or ongoing hyperthy…
DOI: 10.1016/s0022-3476(05)81189-8 -
INFLUENCE OF DOPAMINERGIC, ADRENERGIC AND CHOLINERGIC BLOCKADE AND TRH ADMINISTRATION ON GH RESPONSES TO GRF 1–29
Cholinergic signaling blockade with atropine markedly suppresses GRF 1-29–induced growth hormone release in healthy men, while dopaminergic and adrenergic blockade do not affect it and TRH does not alter GRF effects, implying a hypothalamic cholinergic control of GH secretion.
DOI: 10.1111/j.1365-2265.1986.tb03270.x -
Neuroendocrine effects of endogenous opioid peptides in human subjects: A review
A comprehensive review detailing how endogenous opioid peptides regulate human neuroendocrine function, demonstrating that opioid systems modulate pituitary hormones (LH, ACTH, cortisol, prolactin, GH, TSH), osmoregulation via vasopressin, energy balance and appetite, carbohydra…
DOI: 10.1016/0306-4530(83)90016-1 -
Altered Hormonal Activity in Severely Ill Patients After Injury or Sepsis
Prospective analysis of thyroid and related hormonal changes in 25 severely ill ICU patients (16 septic, 9 multi-system injuries) showing a low T3 syndrome and hypothalamic-pituitary dysregulation; survivors normalized T3/T4 by discharge while non-survivors worsened, with associ…
DOI: 10.1001/ARCHSURG.1984.01390220011003 -
Fasting-Induced Changes in the Hypothalamus–Pituitary–Thyroid Axis
A comprehensive review of how fasting modulates the hypothalamus–pituitary–thyroid axis in humans and rodents, detailing central and peripheral mechanisms—leptin signaling, melanocortin pathways, TRH regulation, pituitary TSH dynamics, and deiodinase-driven peripheral thyroid ho…
DOI: 10.1089/thy.2007.0253 -
Effect of Chronic Treatment with Thyrotropin-Releasing Hormone (TRH) or an Analog of TRH (Linear β-Alanine TRH) on the Hypothalamic-Pituitary-Thyroid Axis
Chronic TRH or an alanine TRH analog modulates the pituitary–thyroid axis in rats and mice in a dose- and species-dependent manner, decreasing TSH and thyroid hormones at higher doses while hypothalamic TRH/LHRH content remains unchanged, suggesting pituitary receptor–level effe…
DOI: 10.1159/000123000 -
Ultra Short-Loop Feedback Control of Thyrotropin Secretion
This study reviews the mechanisms of ultra-short-loop feedback control of thyrotropin (TSH) secretion, highlighting the role of pituitary hormones in regulating their own secretion through hypothalamic and pituitary receptors.
DOI: 10.1089/thy.2004.14.825 -
Iodine Supplementation Decreases Hypercholesterolemia in Iodine-Deficient, Overweight Women: A Randomized Controlled Trial ,
This randomized controlled trial demonstrates that iodine supplementation significantly reduces serum TSH levels and improves lipid profiles, particularly total cholesterol, in overweight, iodine-deficient Moroccan women.
DOI: 10.3945/jn.115.213439 -
The Effect of Vitamin A Supplementation on Thyroid Function in Premenopausal Women
A 4-month, randomized, double-blind trial in 84 Iranian premenopausal women (56 obese and 28 nonobese) found that daily vitamin A supplementation (25,000 IU retinyl palmitate) reduced TSH and increased T3, with T4 decreasing across groups, and a notable drop in retinol-binding p…
DOI: 10.1080/07315724.2012.10720431 -
Thyroid peroxidase in endemic goiter tissue
This study investigates the activity and protein levels of thyroid peroxidase (TPO) in endemic goiter tissues under iodine deficiency and after iodine supplementation, revealing that TPO activity is significantly higher in goiter tissues compared to normal tissues, with implicat…
DOI: 10.1007/bf03349648 -
Effect of Iodine Supplementation on a Pediatric Population with Mild Iodine Deficiency
A two-stage cross-sectional study in Galicia, Spain, found that 11 years of mandatory iodized salt consumption in mildly iodine-deficient schoolchildren did not change overall goiter prevalence but reduced high-degree goiters and subclinical hyperthyroidism, while increasing uri…
DOI: 10.1089/thy.1999.9.1089 -
Effect of 1α-hydroxyvitamin D3 on serum levels of thyroid hormones in hyperthyroid patients with untreated graves' disease
This study investigates the effects of l¢~-hydroxyvitamin D3 on serum thyroid hormone levels in hyperthyroid patients with untreated Graves' disease, finding that its supplementation alongside methimazole significantly enhances the reduction of thyroid hormone levels compared to…
DOI: 10.1016/s0026-0495(97)90214-6 -
Thyrotropin modulates EGF receptor function in porcine thyroid follicle cells
In porcine thyroid follicle cells, thyrotropin (TSH) modulates high-affinity EGF receptor expression via a cAMP-dependent mechanism, increasing EGF binding and enabling synergistic stimulation of DNA synthesis by EGF, suggesting a regulatory cross-talk that could influence thyro…
DOI: 10.1016/0303-7207(85)90153-4 -
Molecular Cloning of the Thyrotropin Receptor
Cloned the dog thyrotropin receptor (TSHR) cDNA via degenerate PCR and cross-hybridization; expression conferred TSH responsiveness to Xenopus oocytes and Y1 cells and TSH binding to COS7 cells, identifying a new G protein-coupled receptor subfamily related to LH-CG receptor.
DOI: 10.1126/science.2556796