Endocrine Methods and the Pituitary-Thyroid Axes

Hormones fall into three chemical classes: amines derived from tyrosine or tryptophan, including catecholamines, thyroid hormones, serotonin, and melatonin; peptides, proteins, and glycoproteins; and steroids. Water-soluble hormones, mostly the peptides and amines, bind surface receptors. Peptides and glycoproteins signal through G-protein-coupled receptors producing cAMP or inositol-trisphosphate and Ca²⁺ second messengers. Growth hormone and prolactin signal through Janus-kinase pathways, and insulin and insulin-like growth factor 1 (IGF-1) through catalytic tyrosine-kinase receptors. Steroids and thyroid hormones cross the membrane and bind intracellular or nuclear receptors that regulate transcription directly. Signal amplification lets picomolar hormone concentrations produce large effects, and receptor defects can cause disease even when the hormone concentration sits inside the reference interval.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Hormone structure also determines the assay format. Large peptide and glycoprotein hormones present several epitopes and suit sandwich designs. Small molecules with one usable epitope, chiefly steroids and thyroid hormones, require competitive designs.

Anterior pituitary hormones are released episodically, so a single random value can neither confirm nor exclude a deficiency of a strongly pulsatile hormone. Growth hormone pulses about every 2 to 4 hours, with the largest pulse shortly after sleep onset. Adrenocorticotropic hormone (ACTH) peaks around 6 to 9 AM, falls to a nadir from about 11 PM to 2 AM, and begins rising again near 2 to 3 AM. Thyroid-stimulating hormone (TSH) rises in the evening, peaks close to sleep onset, and runs about 1 to 3 mIU/L above the afternoon nadir. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion also depends on pulse frequency: slow gonadotropin-releasing hormone (GnRH) pulse frequencies favor FSH secretion and faster frequencies favor LH, and gonadal feedback and inhibin also affect secretion. Reference conditions therefore specify the collection time. When a random value is not interpretable, stimulation or suppression testing is used.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Water-soluble hormones in circulation are subject to rapid degradation and renal filtration. Lipophilic hormones circulate bound to carrier proteins, including albumin, corticosteroid-binding globulin, sex hormone-binding globulin, and thyroxine-binding globulin (TBG). Binding solubilizes the hormone, slows degradation and loss, and provides a reservoir. The free fraction is the immediately diffusible part that binds receptors. Binding is reversible: bound hormone dissociates and replenishes the free pool as tissue uptake proceeds, so the two fractions are in dynamic equilibrium.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Changes in carrier concentration alter the total hormone level without necessarily changing the free fraction. Pregnancy raises TBG through estrogen, so total thyroxine (T₄) rises and free T₄ changes little. Cirrhosis and nephrosis lower albumin.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Free-hormone measurement reduces this confounding but does not remove it. Routine free T₄ and free triiodothyronine (T₃) immunoassays estimate the free fraction indirectly, remain method- and binding-protein dependent, and do not yield interchangeable values across manufacturers. When free-hormone results conflict with the total hormone, the binding proteins, or the clinical setting, equilibrium dialysis or ultrafiltration combined with tandem mass spectrometry is the reference approach, offered by specialized laboratories.^{22 Van Uytfanghe K, et al. Thyroid stimulating hormone and thyroid hormones… 2023.}

In negative feedback, the downstream hormone suppresses its own upstream drivers and maintains a set point. T₄ and T₃ inhibit both thyrotropin-releasing hormone (TRH) and TSH secretion. Positive feedback amplifies a process until a physiologic endpoint or counter-regulation stops it, as in the oxytocin release that cervical stretch reinforces during labor, the midcycle LH surge, and coagulation. Patterns localize disease along an axis:

Sensitive TSH assays have made routine TRH stimulation testing obsolete. TRH testing remains only as an occasional adjunct in unclear central cases, where a blunted or delayed TSH response supports central hypothyroidism but does not separate pituitary from hypothalamic disease.^{33 Persani L, et al. 2018 European Thyroid Association guidelines on the diagnosis… 2018.}

Immunoassays divide by binding format. Competitive designs give patient analyte and labeled analyte a fixed, limited number of antibody sites, so the signal falls as analyte rises. Small molecules with a single epitope, including most steroid hormones and total thyroid hormones, require the competitive format. Sandwich or immunometric designs use excess capture antibody plus a labeled second antibody against a different epitope, so the signal rises with analyte. Peptide and glycoprotein hormones suit this format, and the two-site design typically improves specificity and low-end performance. Performance depends on the particular antibody pair and calibrator, so cross-reactivity and low-end precision cannot be inferred from the design class.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.} Serologic Procedures and Test Results covers the general format mechanics, standard curves, and the shared interference table.

Current platforms. Most routine hormone testing now runs on automated analyzers that read a chemiluminescent signal: acridinium esters, alkaline-phosphatase cleavage of chemiluminogenic substrates, or ruthenium chelates oxidized electrochemically at an electrode. Hormone immunoassays are method-specific because antibody specificity, calibrator materials, and matrix handling differ across manufacturers. Absolute values, reference intervals, and interference profiles therefore differ across methods. TSH calibrators trace to World Health Organization International Standard material. Free T₄ and free T₃ lack that level of standardization, and published between-method biases for free T₄ are large enough to matter clinically.^{22 Van Uytfanghe K, et al. Thyroid stimulating hormone and thyroid hormones… 2023.} Keep serial results for one patient on one method, and interpret them against that method's interval.

Sensitivity and range. Low-end performance is specified as functional sensitivity, the lowest concentration measured with acceptable imprecision over a clinically relevant interval, conventionally a coefficient of variation near 20%. A detection limit alone does not establish that precision. Measuring-range limits and validated dilution protocols come from the assay's instructions and set which results the laboratory can report.^{22 Van Uytfanghe K, et al. Thyroid stimulating hormone and thyroid hormones… 2023.}

Hook effect. An extremely high analyte concentration saturates capture and labeled antibodies separately in a sandwich assay, preventing sandwich formation and producing a falsely low result. Classic settings include prolactin from a macroadenoma, human chorionic gonadotropin (hCG) in gestational trophoblastic disease, and tumor markers. Thyroglobulin and calcitonin results can also be driven falsely low by extreme antigen excess.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.} Serial dilution detects it. In a hooked specimen the measured result rises as dilution relieves antigen excess, and the reportable value comes from the concentration at which two successive dilutions agree after correction. Dilution-corrected results from a specimen without interference agree across the validated interval. When the measured results of a dilution series never agree after correction, do not report the value. Send the specimen for testing by an alternate method.

Heterophile antibodies include human anti-animal antibodies, such as human anti-mouse antibodies from animal exposure or diagnostic antibodies, and rheumatoid factor, an anti-IgG. They can bridge the two antibodies of a sandwich assay, usually producing a falsely high result, or prevent binding and produce a falsely low one. In competitive assays either direction is possible because the interference depends on which reagent interaction the antibody disrupts. An unexpected result discordant with the clinical picture triggers the laboratory's interference investigation: repeat, dilution studies, comparison on a different platform, blocking reagents, or precipitation steps as appropriate to the suspected mechanism. No single check excludes antibody interference, and a dilution series can appear linear even when interference is present.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.}

Biotin interference. In assays that couple reagents through biotin and streptavidin, excess circulating biotin from high-dose supplements competes with the biotinylated reagent for streptavidin. Sandwich assays, including TSH, read falsely low. Competitive assays, including many free T₄, free T₃, and cortisol methods, read falsely high. The direction depends on each design, so look it up in the instructions for the assay in use. FDA's safety communication links high-dose biotin, largely from hair, skin, and nail products, to serious misinterpretations including falsely low troponin results and asks patients to disclose biotin use before testing.^{55 US Food and Drug Administration. Testing for Biotin Interference in In Vitro Diagnostic Devices. 2020.} To resolve suspected biotin interference, test a repeat specimen collected after the manufacturer's washout interval, which can extend to 72 hours or more with high doses and impaired kidney function.

Cross-reactivity and isoforms. Antibodies bind related molecules by structural similarity, so isoforms, prohormones, fragments, metabolites, and analog drugs may be measured as the analyte or may go unrecognized, according to the epitopes the assay targets. Cross-reactant behavior is quantitative and assay-specific. The package insert defines it, sometimes at a single nonrepresentative concentration.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.}

Macroprolactin. Macroprolactin is a high-molecular-weight prolactin-immunoglobulin complex, usually with low bioactivity, that reacts strongly in prolactin immunoassays and inflates the result in otherwise well patients. Polyethylene glycol (PEG) precipitation is the common screening step. Judge recovery after precipitation against method-specific intervals, because PEG also co-precipitates some monomeric prolactin and can occasionally miss macroprolactin. Gel filtration chromatography is the reference characterization when the screen is ambiguous.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.}

Fluorescence labels illustrate signal principles that several endocrine assay families use. A fluorophore absorbs excitation light and re-emits at a longer wavelength. The gap between the peaks is the Stokes shift, and some absorbed energy dissipates as nonradiative relaxation before emission. Right-angle or filtered detection separates the weak emission from the strong excitation beam, although the geometry varies with instrument design.^{66 Hagan AK, et al. Lanthanide-based time-resolved luminescence immunoassays. 2011.}

• Fluorescence immunoassay (FIA) measures fluorescence from a fluorophore-labeled antigen or antibody directly, in either competitive or sandwich format.
• Time-resolved fluorescence immunoassay (TRFIA) uses a lanthanide chelate label, most commonly europium with terbium and samarium also used, whose luminescence decays over microseconds to milliseconds. Background fluorescence from plasma, plastic, and reagents is typically much shorter-lived, on the nanosecond scale. The reader waits a set delay after a brief excitation pulse before it measures, so the background has already decayed. That suppresses the background and raises sensitivity.^{66 Hagan AK, et al. Lanthanide-based time-resolved luminescence immunoassays. 2011.}
• Fluorescence polarization immunoassay (FPIA) is a homogeneous competitive format for small analytes, historically including steroids, thyroid hormones, and therapeutic drugs. Plane-polarized excitation reaches a small fluorophore-labeled antigen. Free label tumbles fast and emits largely depolarized light. Antibody-bound label tumbles slowly and emits polarized light. Patient analyte competes for antibody, so polarization falls as patient analyte rises. Routine endocrine testing has largely moved from FPIA to automated chemiluminescent platforms, and steroid measurement increasingly uses tandem mass spectrometry.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.}

Specimen optics can bias any fluorescence-based signal in either direction. Endogenous background from bilirubin and some drugs adds to the measured signal. Inner-filter absorption, in which hemoglobin or another concentrated constituent absorbs the excitation or emitted wavelength, reduces the signal. True quenching is a separate mechanism: an excited-state energy transfer that hemoglobin can also produce. Scattering from hemolysis and lipemia raises or lowers the measured result depending on the wavelength and geometry, so acceptance thresholds and bias direction come from the validated method.^{44 Ghazal K, et al. Hormone immunoassay interference. 2022.}

Collection time is part of the reference conditions for the rhythmic hormones above, and pituitary-thyroid workups have additional specimen requirements. Acute systemic illness lowers T₃ and, in critical illness, TSH and T₄ as well. A blood sample drawn within several hours of a levothyroxine dose shows a transient free T₄ rise of about 15%, so record the last dose and standardize follow-up sampling.^{77 Jonklaas J, et al. Guidelines for the treatment of hypothyroidism. 2014.} For central hypothyroidism, the European Thyroid Association (ETA) guideline specifies collection before the daily dose or at least 4 hours afterward.^{33 Persani L, et al. 2018 European Thyroid Association guidelines on the diagnosis… 2018.} Record the medication history with the specimen: biotin shifts assay signal, heparin injections release free fatty acids that bias free-hormone estimates, dopamine-agonist drugs suppress prolactin, and dopamine antagonists raise it. Estrogen therapy raises TBG and total T₄. Interpret hemolyzed, lipemic, or icteric specimens against the assay's validated interference limits.

Growth hormone

Growth hormone (GH) is a peptide structurally related to prolactin and human placental lactogen. Somatotrophs, roughly half of anterior pituitary cell mass, produce it under growth hormone-releasing hormone (GHRH) stimulation and somatostatin inhibition, and ghrelin also stimulates release. GH antagonizes insulin peripherally, drives hepatic gluconeogenesis and lipolysis, and mediates growth largely through hepatic IGF-1, insulin-like growth factor 1, formerly called somatomedin C.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

β-blockade removes inhibitory β-adrenergic tone and augments the GH response to provocation. α-blockade blunts it. Drug tables therefore record the receptor pathway as well as the drug class.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Because random GH values reflect pulsatile secretion, an age- and sex-adjusted IGF-1 is the initial laboratory test for suspected acromegaly, and protocol-defined GH suppression after oral glucose evaluates an elevated or equivocal IGF-1. Healthy adults suppress GH below the assay's nadir cutoff. The cutoff is lower in obesity, so the laboratory applies assay- and body-mass-index-specific limits. GH deficiency is established with a validated stimulation test under the local protocol. The stimulated result is interpreted with clinical and imaging findings. IGF-1 is also used to monitor recombinant GH therapy and treated acromegaly.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.}

Prolactin

Prolactin secretion is under tonic hypothalamic dopaminergic inhibition, unlike the other anterior pituitary hormones. Anything that interrupts dopamine delivery or action raises prolactin: dopamine-D₂-antagonist drugs, pituitary stalk compression from a nonsecretory tumor, and severe primary hypothyroidism through TRH-driven stimulation. Prolactinoma is the most common functioning pituitary tumor, with amenorrhea, galactorrhea, and infertility in premenopausal women and mass symptoms or hypogonadism in men and postmenopausal women.^{88 Melmed S, et al. Diagnosis and treatment of hyperprolactinemia. 2011.}

The prolactin concentration narrows the differential but does not establish the cause. Values above 200 to 250 ng/mL strongly suggest a prolactin-secreting adenoma, which usually correlates with tumor size, and values at or above 500 ng/mL in the appropriate setting support macroprolactinoma. Several dopamine antagonists, including risperidone and metoclopramide, can also exceed 200 ng/mL, so review the medication list before the differential. Moderate elevations of 25 to 100 ng/mL have a broad differential: stalk effect, dopamine antagonists, primary hypothyroidism, kidney failure, polycystic ovary syndrome, chest-wall or nipple stimulation, pregnancy, and venipuncture or exercise stress. A large pituitary lesion with only a modest prolactin result suggests a hook effect, so dilute the specimen under the assay's instructions.^{88 Melmed S, et al. Diagnosis and treatment of hyperprolactinemia. 2011.} Screen for macroprolactin wherever an elevated measurement disagrees with the presentation, as described in the interference section. TSH and free T₄ accompany an unexplained prolactin rise because primary hypothyroidism raises TRH, which drives lactotrophs.

Posterior pituitary: oxytocin and AVP

Oxytocin and arginine vasopressin are cyclic nonapeptides synthesized in the supraoptic and paraventricular hypothalamic nuclei and transported down the pituitary stalk.^{1616 Christ-Crain M, et al. The Neurohypophysis. 2025.} Oxytocin participates in parturition, where cervical stretch reinforces its release, and mediates the milk-ejection reflex during nursing.^{1616 Christ-Crain M, et al. The Neurohypophysis. 2025.} AVP acts at renal V₂ receptors to insert aquaporin-2 water channels, and at vascular V_1a receptors to constrict vessels.^{1616 Christ-Crain M, et al. The Neurohypophysis. 2025.} Its endothelial V₂ action also contributes to release of von Willebrand factor and factor VIII.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} Hypothalamic osmoreceptors trigger AVP release near a plasma osmolality of about 284 mOsm/kg, and a mean arterial pressure fall of roughly 5 to 10% produces graded nonosmotic release that can dominate osmotic control in hypovolemia.^{1616 Christ-Crain M, et al. The Neurohypophysis. 2025.} AVP itself is unstable and rapidly cleared, so serum sodium and osmolality, urine osmolality, and the stable C-terminal fragment copeptin are the laboratory measurements used.^{99 Atila C, et al. Arginine vasopressin deficiency. 2024.}

Arginine-vasopressin deficiency, formerly central diabetes insipidus, and arginine-vasopressin resistance, formerly nephrogenic diabetes insipidus, both cause dilute polyuria with compensatory polydipsia, and primary polydipsia presents the same way. The laboratory starts with paired serum sodium and osmolality against urine osmolality. When the pattern stays unclear, a supervised hypertonic-saline stimulated copeptin protocol has the best discrimination where validated. Where copeptin is unavailable, water-deprivation and desmopressin protocols follow validated local pathways.^{99 Atila C, et al. Arginine vasopressin deficiency. 2024.} A therapeutic desmopressin response does not establish the diagnosis, and desmopressin can cause rapid dilutional hyponatremia in primary polydipsia, so dynamic testing is done only under a validated protocol.

Hypopituitarism

Hypopituitarism may involve one anterior axis or all of them, the latter called panhypopituitarism. In adults, pituitary adenomas are the most common cause, usually through compression of functioning gland. Surgery, radiation, apoplexy, Sheehan syndrome after severe postpartum hemorrhage, hypophysitis, infiltrative disease, traumatic brain injury, and congenital defects account for the rest. Stalk compression or destruction interrupts dopaminergic delivery and raises prolactin, a useful laboratory sign. Laboratory evaluation follows the affected axes: free T₄ with TSH, an 8 to 9 am cortisol for the adrenal axis, age-adjusted IGF-1 with dynamic GH testing when indicated, and LH and FSH interpreted with sex steroids and reproductive status.^{1010 Fleseriu M, et al. Hormonal replacement in hypopituitarism in adults. 2016.}

Synthesis and control. Iodine trapping, organification, and coupling in thyroid follicular cells yield mostly T₄, a prohormone. Peripheral deiodinases then convert T₄: outer-ring (5′) deiodination converts roughly one third of T₄ turnover into active T₃, and inner-ring (5) deiodination converts a similar share, about 45%, into inactive reverse T₃, with much of the remainder conjugated and excreted. About 80% of circulating T₃ comes from this peripheral conversion, and T₃ binds receptors several-fold more avidly than T₄, with the ratio varying by tissue and endpoint. Only about 0.03% of circulating T₄ and 0.3% of T₃ are unbound. TBG carries roughly 70% of the T₄, and transthyretin and albumin carry most of the rest.^{11 Rifai N, et al. Tietz Textbook of Laboratory Medicine. 2023.} T₄ and T₃ suppress TRH and TSH secretion, so in an intact axis TSH tracks the free-hormone signal the pituitary receives, which is why interpretation of a thyroid panel pairs TSH with free T₄.^{22 Van Uytfanghe K, et al. Thyroid stimulating hormone and thyroid hormones… 2023.}

Automated third-generation TSH immunoassays reach functional sensitivity near 0.01 mIU/L, enough to separate suppressed from low-normal TSH and to detect the subnormal values of subclinical disease. The method's functional sensitivity determines whether a low result can be interpreted reliably. Its detection limit alone cannot establish this.

Pregnancy changes thyroid test results in two ways. Chorionic gonadotropin stimulates the TSH receptor and lowers TSH, most noticeably toward the end of the first trimester. Rising TBG increases total T₄. The 2026 ATA guideline favors laboratory- and trimester-specific TSH and free T₄ intervals. When a pregnancy-specific free T₄ interval is unavailable, gestational-week-adjusted total T₄ is one alternative, interpreted with the platform's limitations. Free T₄ immunoassay behavior varies by method and stage of pregnancy, so use the same assay for serial follow-up.^{1414 Korevaar TIM, et al. American Thyroid Association 2026 guidelines for thyroid disease… 2026.}

Newborn screening for congenital hypothyroidism uses a dried blood spot collected early in life, most programs targeting 24 to 48 hours after the postnatal TSH surge has peaked. TSH-primary programs detect primary thyroid failure most sensitively. Programs that measure T₄ first, with reflex TSH, add detection of low-T₄ states. Preterm, low-birth-weight, and neonatal intensive care unit (NICU) infants mount a blunted, delayed TSH rise, so programs repeat screening in them. An abnormal screen is confirmed with a serum TSH and free T₄.^{1515 Rose SR, et al. Congenital hypothyroidism. 2023.} For every hormone in this lesson, know the collection time, the binding proteins, and the assay method before you interpret the result.