Endocrinologists order a wide range of tests, from routine blood draws measuring hormone levels to specialized imaging, biopsies, and provocative challenge tests that push a gland to reveal whether it is working properly. The specific lineup depends on which gland or hormone system is under suspicion, but nearly every endocrine workup starts with blood tests and expands from there based on what those initial results show. Because the endocrine system touches almost every organ, the diagnostic toolkit is broader than most patients expect.
Blood Tests Are the Foundation
The single most common thing an endocrinologist does is order blood work. Hormones circulate in the bloodstream, so a well-timed blood draw can reveal whether a gland is overproducing, underproducing, or responding abnormally to signals from the brain. Thyroid-stimulating hormone (TSH), free T4, and free T3 are typically the first tests for suspected thyroid problems. Fasting glucose and hemoglobin A1c are standard for diabetes evaluation. Cortisol, testosterone, estradiol, prolactin, insulin, and parathyroid hormone each come into play depending on your symptoms.
Timing matters more than many people realize. Testosterone levels, for instance, peak in the early morning and drop significantly by afternoon. Research supports restricting testosterone measurements to morning hours in both younger and older men to avoid misleadingly low readings.1The Journal of Clinical Endocrinology & Metabolism. The Effect of Diurnal Variation on Clinical Measurement of Serum Testosterone and Other Sex Hormone Levels in Men Cortisol follows a similar daily rhythm, which is why some cortisol tests specifically call for late-night sampling. If your endocrinologist is particular about when you show up for your blood draw, this is why.
Thyroid Testing Beyond Blood Work
Thyroid disorders are among the most common reasons people see an endocrinologist, and the workup can go well beyond a TSH level. If a nodule is found on your thyroid, ultrasound is usually the first imaging step. It shows the size, shape, and internal characteristics of the nodule and helps the doctor decide whether a biopsy is needed. Fine-needle aspiration biopsy (FNAB) is the standard method for sampling a suspicious nodule. A thin needle is guided into the nodule under ultrasound, and cells are withdrawn for examination under a microscope.2PubMed. The thyroid: review of imaging features and biopsy techniques with radiologic-pathologic correlation If the initial biopsy comes back inconclusive, a core biopsy using a slightly larger needle may follow.
Nuclear thyroid scans, also called scintigraphy, used to be a frontline test for evaluating nodules. The patient swallows or is injected with a small amount of radioactive iodine, and a camera detects which parts of the thyroid are absorbing it. “Hot” nodules that take up lots of iodine are almost always benign, while “cold” nodules that do not absorb iodine carry a higher risk of malignancy. With the widespread adoption of ultrasound and FNAB, the role of thyroid scanning in the initial evaluation of nodules has diminished, though it remains useful in specific scenarios like confirming hyperthyroidism or evaluating a gland with multiple nodules.3Endocrinology and Metabolism Clinics of North America. RADIOIODINE UPTAKE AND THYROID SCINTISCANNING
When a biopsy of a thyroid nodule falls into an indeterminate category, molecular testing has become increasingly important. These gene-panel tests look for mutations associated with thyroid cancer and can help clarify whether a nodule is likely benign or malignant, reducing unnecessary surgeries. Advances in next-generation sequencing and micro-RNA platforms over the past two decades have refined this capability considerably.4PubMed Central. Molecular diagnostics in the evaluation of thyroid nodules: Current use and prospective opportunities One study analyzing an 18-gene panel found that the most common mutation in thyroid nodule samples was BRAF, detected in roughly three-quarters of cases with mutations, with RAS, RET/PTC fusions, and TERT promoter mutations making up most of the remainder.5PubMed Central. Molecular testing raises thyroid nodule fine needle aspiration diagnostic value The goal is to better distinguish benign from malignant nodules so that treatment decisions can be more precise.6PubMed Central. Molecular testing in indeterminate thyroid nodules: an additional tool for clinical decision-making
Diabetes and Pancreatic Function Tests
Most people associate endocrinology with diabetes, and the testing here is layered. Fasting blood glucose and hemoglobin A1c are the bread-and-butter screening tools, but an endocrinologist often digs deeper. The oral glucose tolerance test (OGTT) involves drinking a standard glucose solution and then having blood drawn at intervals over the next two to three hours. This shows how well your body handles a sugar load in real time.
C-peptide is a molecule released in equal amounts alongside insulin when the pancreas produces it. Measuring C-peptide tells the endocrinologist how much insulin your pancreas is actually making, which is critical for distinguishing between type 1 and type 2 diabetes and for gauging how much beta-cell function remains. C-peptide can be measured as a single fasting level, or it can be tracked across multiple time points during an OGTT to get a more dynamic picture of insulin secretion.7PubMed Central. A Practical Review of C-Peptide Testing in Diabetes
When the question is whether diabetes has an autoimmune component, antibody tests enter the picture. Glutamic acid decarboxylase antibody (GADA) is the most commonly used marker for autoimmune diabetes. A positive GADA result in someone who looks clinically like they have type 2 diabetes may indicate latent autoimmune diabetes in adults (LADA), a condition that progresses toward insulin dependence more slowly than classic type 1 but still reflects immune-mediated destruction of insulin-producing cells.8Frontiers in Endocrinology. Islet Cell Associated Autoantibodies and C-Peptide Levels in Patients with Diabetes and Symptoms of Gastroparesis
For ongoing management rather than diagnosis, endocrinologists increasingly use continuous glucose monitors (CGMs). Professional CGMs can be placed on the arm for up to 14 days, recording interstitial glucose readings every 15 minutes. In some configurations the patient never sees the data in real time; the sensor stores everything for the doctor to review afterward, giving an unbiased picture of glucose patterns during everyday life.9PubMed Central. Professional continuous glucose monitoring and endocrinology eConsult for adults with type 2 diabetes in primary care: Results of a clinical pilot program This is especially useful when A1c readings and fingerstick logs do not tell the whole story.
Adrenal Gland Workups
The adrenal glands sit on top of the kidneys and produce cortisol, aldosterone, and adrenaline-related hormones. When an endocrinologist suspects the adrenals are misbehaving, the testing gets creative because simple blood levels often are not enough.
For suspected Cushing syndrome (too much cortisol), first-line screening tests include the overnight dexamethasone suppression test, the 24-hour urine free cortisol collection, and the late-night salivary cortisol test. The dexamethasone test works by giving you a small dose of a synthetic steroid at bedtime and checking your cortisol the next morning; in a healthy person, that dose should suppress cortisol production, and failure to suppress suggests an abnormal cortisol source. Because cortisol production varies significantly from day to day, the urine and salivary tests should each be performed at least twice before drawing conclusions.10The Journal of Clinical Endocrinology & Metabolism. Approach to the Patient: Diagnosis of Cushing Syndrome
For adrenal tumors that produce adrenaline-like hormones (pheochromocytomas), the key test measures fractionated metanephrines and catecholamines, either in blood or in a 24-hour urine collection. These biochemical tests should always come before any imaging. Separately, when resistant high blood pressure or unexplained low potassium raises the possibility of excess aldosterone production, an aldosterone-to-renin ratio is the go-to screening test.11PubMed. Adrenal causes of hypertension: pheochromocytoma and primary aldosteronism
Pituitary Testing and Stimulation Challenges
The pituitary gland is a pea-sized structure at the base of the brain that acts as a master controller, sending signals to the thyroid, adrenals, and gonads. Testing pituitary function often involves what are called dynamic or provocative tests, because a single hormone level drawn at one moment may not reveal the problem.
Acromegaly, caused by excess growth hormone (GH), is a good example. Screening starts with measuring insulin-like growth factor 1 (IGF-1), which reflects average GH activity. If IGF-1 is elevated, the next step is a glucose tolerance test. In a healthy person, drinking a glucose load suppresses GH; in acromegaly, GH fails to drop. This lack of suppressibility confirms the diagnosis.12Endocrinology and Metabolism Clinics of North America. Diagnosis and Endocrine Testing in Acromegaly
When the pituitary itself needs to be located as the source of excess hormone production, venous sampling can provide the answer. This invasive procedure involves threading a catheter into veins near the pituitary and measuring hormone levels at the source. In cases of ACTH-dependent Cushing syndrome, bilateral inferior petrosal sinus sampling (BIPSS) is considered the gold standard for confirming that the pituitary, rather than another site, is the culprit.13PubMed. A comparison of bilateral internal jugular vein sampling with bilateral inferior petrosal sinus sampling at Mayo Clinic for the subtype evaluation of corticotropin-dependent cushing syndrome Venous sampling is also used in the workup of primary aldosteronism and hyperparathyroidism when imaging alone cannot pinpoint the problem.14PubMed Central. Role of Venous Sampling in the Diagnosis of Endocrine Disorders
Reproductive Hormone Panels
Endocrinologists frequently evaluate reproductive hormones in both men and women. For women with irregular periods or suspected infertility, a standard panel includes TSH, prolactin, FSH, and estradiol. Depending on the clinical picture, additional tests for DHEAS, testosterone, and LH may be added.15The Journal of Clinical Endocrinology & Metabolism. Evaluation of Female Fertility—AMH and Ovarian Reserve Testing Anti-Müllerian hormone (AMH) has become a particularly valuable marker for ovarian reserve, with advantages over older measures like day-3 FSH because it can be drawn at any point in the menstrual cycle and remains relatively stable.16The Journal of Clinical Endocrinology & Metabolism. Clinical Utility of Measurement of Anti-Müllerian Hormone in Reproductive Endocrinology Women undergoing fertility treatments typically get AMH testing alongside an ultrasound-based antral follicle count to estimate how many eggs remain available for stimulation.
For men, the reproductive workup centers on testosterone (drawn in the morning, as noted earlier), along with FSH and LH to determine whether low testosterone is coming from the testes or from the brain’s signaling pathway. Prolactin and sometimes estradiol are measured to rule out other causes of hormonal disruption. These hormones, whether in men or women, interact in complex feedback loops, which is why interpreting the results often requires an endocrinologist rather than a quick glance at reference ranges.17Current Medicine Research and Practice. Clinical significance of reproductive hormones
Bone Density and Mineral Metabolism
Endocrinologists handle a significant share of osteoporosis care, and the primary diagnostic tool is the DEXA scan (dual-energy X-ray absorptiometry). This low-radiation scan of the hip and spine generates a T-score that compares your bone density to a reference population.18PubMed Central. Correlation of vitamin D, bone mineral density and parathyroid hormone levels in adults with low bone density Blood tests for calcium, phosphorus, vitamin D, and parathyroid hormone (PTH) typically accompany the scan. Elevated PTH with high calcium, for example, points toward hyperparathyroidism, while low vitamin D paired with elevated PTH suggests the body is compensating for inadequate vitamin D rather than harboring a parathyroid problem.
Neuroendocrine Tumor Diagnostics
Neuroendocrine tumors (NETs) are uncommon but increasingly recognized, and endocrinologists are often involved in their evaluation. A specialized PET/CT scan using gallium-68 DOTATATE has become the preferred imaging modality for well-differentiated NETs. The radioactive tracer binds to somatostatin receptors that are abundant on these tumors, making them light up on the scan with high sensitivity. DOTATATE PET/CT is used for initial diagnosis, for finding tumors whose primary site is unknown, and for selecting patients who might benefit from targeted radioligand therapy.19PubMed. Neuroendocrine Tumor Diagnosis and Management: (68)Ga-DOTATATE PET/CT
Alongside imaging, blood markers play a supporting role. Chromogranin A (CgA) is a protein secreted by neuroendocrine cells, and elevated levels in the blood raise suspicion for a NET. In patients with gastroenteropancreatic NETs, elevated CgA and larger tumor size are independently associated with metastatic disease on DOTATATE imaging.20PubMed. Larger Tumor Size and Elevated Serum Chromogranin A Levels Predict Metastatic Disease on DOTATATE Imaging in Patients with Gastroenteropancreatic Neuroendocrine Tumors CgA is not perfect as a standalone test, since it can be falsely elevated by acid-reducing medications and other conditions, but combined with imaging it adds useful prognostic information.
When Pregnancy Changes the Reference Ranges
Pregnancy alters nearly every endocrine axis, and this creates a specific challenge: the normal ranges that apply outside of pregnancy do not apply during it. Thyroid function is the clearest example. Rising estrogen during pregnancy increases the levels of thyroid-binding proteins, which in turn raises total T4 to roughly 1.5 to 2 times prepregnancy values.21PubMed Central. Trimester-specific reference ranges for thyroid hormones in pregnant women TSH also shifts, typically dropping in the first trimester because of stimulation from the pregnancy hormone hCG. If a lab uses standard non-pregnant reference ranges for a pregnant woman, it can easily misclassify normal thyroid function as abnormal or miss genuine thyroid disease.
For this reason, trimester-specific reference intervals have become standard practice in endocrine care during pregnancy. These ranges vary not just by trimester but by population and by the assay method used, so the same TSH level can be interpreted differently depending on the lab, the woman’s gestational week, and even her geographic region.22PubMed Central. Trimester-Specific Reference Intervals for Thyroid Function Parameters in Indian Pregnant Women during Final Phase of Transition to Iodine Sufficiency Getting thyroid management right during pregnancy matters because untreated thyroid insufficiency is linked to adverse pregnancy outcomes and effects on fetal brain development.23PubMed Central. Thyroid function testing in pregnancy and thyroid disease: trimester-specific reference intervals
Pediatric Endocrine Testing Has Its Own Quirks
Children are not small adults when it comes to endocrine diagnostics, and one test that illustrates this well is the bone age X-ray. A single X-ray of the left hand and wrist is compared against a reference atlas to estimate skeletal maturity. Endocrinologists use this to evaluate children with early or delayed puberty, short stature, or premature adrenarche (early appearance of pubic hair or body odor without other signs of puberty). If bone age is significantly advanced beyond chronological age, it may signal an underlying hormonal issue that warrants further testing.
However, the most widely used reference atlas was developed in the 1930s from X-rays of about a thousand white, middle-to-upper socioeconomic status children in North America. Research has shown that children from diverse ethnic and racial backgrounds may have normal skeletal maturation patterns that diverge from this atlas, raising concerns that the standard can lead to over-investigation in some groups and missed diagnoses in others.24Pediatric Research. Bone age evaluation in an ethnically diverse cohort of children with premature adrenarche An endocrinologist aware of this limitation will interpret bone age in context rather than treating a single number as definitive.
Biotin Supplements and Lab Interference
Here is a practical pitfall that catches patients and even some doctors off guard. Biotin, also sold as vitamin B7, is a popular supplement marketed for hair and nail growth. Many common lab assays that endocrinologists rely on use biotin-based chemistry in their testing platforms, and high levels of biotin in your blood can directly interfere with results. Depending on the test design, biotin can push results falsely high or falsely low. Thyroid tests, cortisol, testosterone, and troponin (a cardiac marker) are among those affected.25Endocrine Practice. Biotin Interference with Routine Clinical Immunoassays: Understand the Causes and Mitigate the Risks
The problem has grown alongside the popularity of biotin supplements, with case reports documenting clinically significant lab errors that led to unnecessary treatment or delayed diagnoses.26PubMed Central. Clinically Significant Lab Errors due to Vitamin B7 (Biotin) Supplementation: A Case Report Following a Recent FDA Warning If you take biotin and your endocrinologist asks you to stop it before blood work, this is the reason. Most guidance suggests discontinuing biotin at least 48 to 72 hours before testing, though high-dose users may need longer. It is a small thing that can make a big difference in whether your results are trustworthy.