The Adrenal, Gonadal, and Parathyroid-Vitamin D Axes

Endocrine testing spans three systems: the adrenal, gonadal, and parathyroid-vitamin D axes. The adrenal cortex makes aldosterone, cortisol, and androgens in three zones with separate control systems. The adrenal medulla and sympathetic nervous system split catecholamine production. The gonadal axes follow pulsatile GnRH into sex-steroid testing. Parathyroid hormone with vitamin D governs calcium and phosphate. Each interpretation depends on timing and on which node of the axis fails.¹

The adrenal cortex

Zonation and steroidogenesis

The adrenal cortex has three zones, each of which synthesizes distinct end-products from the shared cholesterol precursor. The outer zona glomerulosa, about 10% of cortical mass, produces aldosterone. Aldosterone is the mineralocorticoid that promotes sodium retention, potassium excretion, and blood-pressure maintenance; the renin-angiotensin system controls it rather than ACTH. The middle zona fasciculata, about 75%, produces cortisol. Cortisol is the glucocorticoid of glucose homeostasis and stress response. The HPA axis controls it: CRH drives ACTH, and cortisol closes the loop by suppressing both. The inner zona reticularis, about 15%, produces adrenal androgens, DHEA, DHEA-S, and androstenedione. Adrenal androgens are the dominant androgen source in women at 40 to 65% of daily testosterone equivalent versus under 5% in men. ACTH drives cholesterol import into mitochondria and the rate-limiting conversion to pregnenolone through CYP450 side-chain cleavage; zone-specific microsomal enzymes finish each lineage.¹

Congenital adrenal hyperplasia

Congenital adrenal hyperplasia results from inherited enzyme blocks in this pathway and produces predictable syndromes according to the affected enzyme:

Nonclassic CAH can present with hirsutism, menstrual irregularity, and infertility.

Primary aldosteronism

Primary aldosteronism is aldosterone secretion that cannot be suppressed by salt or volume loading. It produces hypertension, hypokalemia, and metabolic alkalosis, and may result from an aldosterone-producing adenoma, bilateral hyperplasia, familial glucocorticoid-remediable aldosteronism, or rarely adrenocortical carcinoma. Screening requires suppressed renin with inappropriately high aldosterone. Numeric aldosterone-to-renin-ratio and aldosterone cutoffs depend on assay, units, posture, sodium status, potassium, medications, and local protocol. The 2025 Endocrine Society guideline suggests screening hypertensive individuals with morning, seated aldosterone and renin testing alongside serum potassium, and treats the ratio thresholds as guidance values with local assay-specific ranges preferred where available.²

Adrenal insufficiency

Adrenal insufficiency may be primary, secondary, or tertiary. Primary insufficiency reflects adrenal gland failure, most commonly from autoimmune destruction and also from infection or hemorrhage. Secondary insufficiency reflects inadequate pituitary ACTH. Tertiary insufficiency reflects inadequate hypothalamic CRH, most commonly from chronic exogenous glucocorticoid suppression. Because cortisol follows a strong diurnal rhythm, sampling timing matters. An 8 am cortisol below 3 µg/dL strongly suggests insufficiency when interpreted using the laboratory’s validated interval. The confirmatory ACTH (cosyntropin) stimulation test measures baseline and post-stimulation cortisol; pass criteria follow the local protocol and assay.³

Cushing’s syndrome

Hypercortisolism presents with central obesity, dorsocervical and facial fat deposition, skin atrophy, striae, and bruising, androgen-excess signs in women, proximal weakness, and reduced bone density. Cortisol excess may be ACTH-independent, with an adrenal tumor and low ACTH from feedback, or ACTH-dependent, including pituitary Cushing’s disease in roughly 70% of cases and ectopic sources such as small cell lung cancer. After excluding exogenous glucocorticoid use, initial screening uses one appropriate high-accuracy test: 24-hour urine free cortisol with at least two collections, late-night salivary cortisol with at least two samples, or dexamethasone suppression, either the 1-mg overnight test with a cortisol above 1.8 µg/dL (50 nmol/L) suggesting failure of suppression or a 2-mg 48-hour variant. An abnormal result is confirmed with another recommended test, and two concordantly abnormal different tests support the diagnosis.³

Cortisol measurement. About 90% of circulating cortisol is protein-bound, mostly to transcortin or corticosteroid-binding globulin, with the rest to albumin; only the free 10% is active. Immunoassays remain common for routine cortisol measurement but can cross-react with steroid metabolites or synthetic glucocorticoids, with exact cross-reactivity assay-specific; prednisolone cross-reactivity has been reported near 24% on one platform, and dexamethasone cross-reacts minimally in many assays. LC-MS/MS provides greater analytical specificity and is increasingly used. Reference values run roughly 5 to 25 µg/dL, 140 to 690 nmol/L, at 8 to 10 am, falling to roughly 3 to 12 µg/dL, 80 to 330 nmol/L, by late afternoon, and the interval is assay- and laboratory-specific. The diurnal swing is wide enough that single random values are best interpreted through dynamic stimulation or suppression testing. Urinary free cortisol reflects the small filtered and unbound fraction, integrates total daily secretion, and is useful for sporadic hypercortisolism; it is unaffected by altered hepatic cortisol metabolism, though renal impairment invalidates it.⁴

Adrenal androgens

DHEA and DHEA-S are regulated by ACTH, with no dedicated tropic hormone. Elevated DHEA and DHEA-S point to an adrenal source of androgen excess because the gonads contribute under 10% of the circulating pool. Excess causes virilization and precocious puberty in children, hirsutism, acne, and menstrual irregularity in women, and feminization through peripheral aromatization in men.

The adrenal medulla and catecholamines

The synthetic pathway proceeds from phenylalanine through tyrosine and DOPA to dopamine in the cytosol, then to norepinephrine in storage vesicles. Cytosolic cortisol-dependent PNMT converts norepinephrine to epinephrine, which is then re-stored. The adrenal medulla is about 80% epinephrine. The sympathetic nervous system is about 9:1 norepinephrine-dominant because 98% of peripheral norepinephrine arises from postganglionic neurons rather than the medulla. Adrenal insufficiency, with low cortisol and low PNMT drive, shifts the ratio sharply toward norepinephrine. Nonneuronal COMT and neuronal MAO degrade catecholamines to metanephrines and normetanephrines and ultimately vanillylmandelic acid.⁵

Pheochromocytoma and paraganglioma (PPGL). These rare catecholamine-secreting tumors arise from chromaffin cells in the adrenal medulla in about 90% of cases or from extra-adrenal paraganglia. About 35% of paragangliomas are malignant. They occur in 0.2 to 0.6% of hypertensive patients. The classic triad is episodic headache, palpitations, and sweating, often with paroxysmal hypertension. First-line biochemical testing is plasma free metanephrines, with sensitivity 96 to 100% and specificity 85 to 89% and lower specificity in older patients, or 24-hour urine fractionated metanephrines, with sensitivity and specificity both around 98%. Mass-spectrometric or electrochemical detection is preferred analytically, and blood drawn fully supine after at least 30 minutes of rest uses supine reference ranges. An equivocal result, less than threefold above the upper limit, may prompt a clonidine suppression test, which evaluates noradrenergic secretion and is reserved for persistent borderline normetanephrine elevations.⁵

The gonadal axes

Testes

The SRY-gene product diverts the bipotential gonad toward testis formation by about 7 weeks’ gestation. hCG and then LH drive Leydig cells to produce testosterone; Sertoli cells produce anti-Müllerian hormone, which regresses the Müllerian ducts. Pulsatile GnRH drives LH on Leydig cells and FSH on Sertoli cells through the hypothalamic-pituitary-testicular axis. This axis drives puberty and adult spermatogenesis. LH, FSH, TSH, and hCG share an alpha subunit, and the beta subunit confers specificity. Beta-hCG assays therefore underpin trophoblastic and germ-cell tumor testing in the marker module. Intracellularly, testosterone is metabolized either to dihydrotestosterone through 5α-reductase, which binds receptors with higher affinity and dominates in skin and prostate, or to estradiol by aromatization in adipose tissue. Testosterone peaks around 6 am and troughs near midnight, so morning sampling matches most reference intervals.¹

Ovaries

Absent a Y chromosome, the bipotential gonad defaults to ovarian development. During the follicular phase, FSH stimulates follicular growth and estradiol rises. The estrogen peak triggers a positive-feedback LH surge, followed by ovum release about 36 hours later. During the luteal phase, the corpus luteum produces thermogenic progesterone, the basis of basal-temperature ovulation tracking. Absent fertilization, the corpus luteum regresses about 14 days after ovulation, progesterone and estrogen fall, and menses begins the next cycle.

Amenorrhea is evaluated after excluding pregnancy; TSH, prolactin, FSH, and estradiol then separate the endocrine patterns:

Precocious puberty results from premature sex-steroid exposure. Turner syndrome, 45,X or mosaic, causes hypergonadotropic hypogonadism without the vasomotor flushing of typical menopause. Menopausal hormone-therapy risks depend on regimen and patient factors: combined estrogen-progestogen therapy shows a clearer association with increased breast-cancer risk, while estrogen-alone therapy after hysterectomy did not show the same increase in the Women’s Health Initiative trial, although systemic therapy can increase stroke and thromboembolic risk.⁶

Parathyroid hormone, vitamin D, and bone

PTH. Parathyroid glands, typically four in number, secrete PTH in response to falling ionized calcium, which parathyroid calcium-sensing receptors detect. PTH raises calcium through osteoclastic bone resorption, increased renal tubular calcium reabsorption, and enhanced renal 1α-hydroxylation of 25-hydroxyvitamin D to active calcitriol, which drives intestinal calcium absorption. PTH simultaneously increases renal phosphate excretion.¹

Vitamin D activation. Cutaneous UVB converts 7-dehydrocholesterol to cholecalciferol, D₃. Hepatic 25-hydroxylase produces 25-hydroxyvitamin D, the best index of body stores. PTH-driven renal 1α-hydroxylase completes activation to 1,25-dihydroxyvitamin D, calcitriol, which drives active intestinal calcium absorption and cooperates with PTH on bone; only about 5 to 10% of calcium absorption is passive. Elevated phosphate stimulates fibroblast growth factor 23 from osteocytes. FGF23 inhibits renal 1α-hydroxylation and suppresses calcitriol, a mechanism central to the bone-mineral disturbances of chronic kidney disease.

Primary hyperparathyroidism is autonomous PTH overproduction and the most common cause of outpatient hypercalcemia. It is usually caused by a single adenoma, more than 80% of cases, less often by multigland hyperplasia in 5 to 10%, and rarely by carcinoma. The biochemical signature is hypercalcemia with inappropriately normal or elevated PTH, hypophosphatemia from PTH-driven phosphaturia, high-normal or elevated 1,25(OH)₂D, elevated urinary calcium excretion, and, with prolonged phosphate-for-chloride exchange, a mild hyperchloremic metabolic acidosis. Intraoperative PTH monitoring, possible because PTH’s half-life is under 5 minutes, confirms adenoma removal by a sharp post-excision drop.¹

Secondary hyperparathyroidism is an appropriate PTH rise driven by chronic hypocalcemia or low vitamin D. It is most consequential in chronic kidney disease, where phosphate retention, FGF23 elevation, and impaired calcitriol synthesis promote continued parathyroid stimulation and hyperplasia. When the process becomes autonomous, it is termed tertiary hyperparathyroidism.

Familial hypocalciuric hypercalcemia, often caused by a heterozygous inactivating CASR variant that raises the calcium set point, mimics primary hyperparathyroidism biochemically with mild hypercalcemia and mild PTH elevation. FHH is favored by a calcium-to-creatinine clearance ratio below 0.01, but results overlap with primary hyperparathyroidism; interpret urine calcium with kidney function, vitamin D status, calcium intake, and medications, and use family history or genetic testing when uncertainty remains.

PTH-related protein is secreted by many solid tumors, shares PTH’s N-terminal receptor activity in bone resorption and renal tubular reabsorption, and escapes calcium feedback suppression. The resulting unchecked, often severe hypercalcemia causes most humoral hypercalcemia of malignancy. PTH and PTHrP assays do not cross-react, so PTH is characteristically low or suppressed in PTHrP-driven hypercalcemia, distinguishing it biochemically from primary hyperparathyroidism. The tumor-marker module covers PTHrP as a tumor marker.⁷

Hypoparathyroidism most often follows inadvertent parathyroid removal during neck surgery, including thyroidectomy with lymph-node dissection. Hypervitaminosis D follows excessive supplementation or extrarenal 1α-hydroxylation by granulomatous tissue in sarcoidosis or tuberculosis.

Metabolic bone disease. Rickets in growing bone and osteomalacia in mature bone both result from defective mineralization, usually caused by vitamin D deficiency. Less often, genetic defects in vitamin D metabolism or receptor function require treatment with the active metabolite, calcitriol, where native vitamin D cannot bypass the block. Bone turnover is tracked with resorption markers, including hydroxyproline, N-telopeptide and C-telopeptide, pyridinium crosslinks, and tartrate-resistant acid phosphatase, and formation markers, including bone-specific alkaline phosphatase, osteocalcin, and procollagen N-terminal extension peptides.¹