Clinical Lab Diagnosis - Endocrine / Thyroid - AI Generated

A comprehensive set of vocabulary flashcards summarizing key hormones, enzymes, antibodies, tests, disorders, and clinical concepts related to thyroid and parathyroid physiology and pathology.

Thyroid Gland

Endocrine organ whose primary functions relate to regulation of metabolism via secretion of T3 and T4.

Thyroxine (T4)

Major thyroid hormone secreted; mostly converted to T3 in tissues; measured as free or total to assess thyroid function.

Tri-iodothyronine (T3)

Active thyroid hormone formed largely by peripheral conversion of T4; drives metabolic effects.

Free T4 (FT4)

Unbound fraction of thyroxine in circulation; best reflects thyroid output when interpreted with TSH.

Total T4

Sum of bound and unbound thyroxine; affected by binding-protein levels (TBG, albumin).

Free T3

Unbound triiodothyronine; measurement often unreliable but can aid hyperthyroid assessment.

Total T3

Sum of bound and unbound triiodothyronine; rises early in hyperthyroidism, last to fall in hypothyroidism.

Reverse T3 (rT3)

Biologically inactive isomer of T3 produced from T4; not useful for routine hypothyroid evaluation.

Thyrotropin-Releasing Hormone (TRH)

Hypothalamic peptide that stimulates pituitary secretion of TSH.

Thyroid-Stimulating Hormone (TSH)

Pituitary glycoprotein (thyrotropin) that stimulates thyroid synthesis and release of T4 and T3.

Hypothalamic-Pituitary-Thyroid Axis

Feedback loop in which TRH→TSH→T4/T3 regulate metabolic homeostasis.

Thyroglobulin (TG)

Large iodinated protein in thyroid follicles serving as precursor scaffold for T3 and T4 synthesis.

Monoiodotyrosine (MIT)

Tyrosine residue iodinated at one position within thyroglobulin during hormone synthesis.

Diiodotyrosine (DIT)

Tyrosine residue iodinated at two positions; coupling of MIT and DIT forms thyroid hormones.

Thyroid Peroxidase (TPO)

Enzyme that oxidizes iodide, organifies tyrosine, and couples iodotyrosines to make T3/T4.

Propylthiouracil (PTU)

Antithyroid drug that inhibits TPO and peripheral conversion of T4 to T3.

Perchlorate

Ion that blocks sodium-iodide symporter, inhibiting iodide uptake into thyroid follicular cells.

Thiocyanate

Competitive inhibitor of iodide transport into thyroid, reducing hormone synthesis.

Deiodinase

Enzyme family that removes iodine from thyroid hormones, converting T4 to T3 or rT3.

Radioactive Iodine Uptake (RAIU)

Diagnostic test measuring thyroid uptake of oral radioiodine at 2-24 h to differentiate causes of hyperthyroidism.

Thyroid Scan

Imaging study performed with radioisotope to visualize thyroid anatomy and function.

Thyroid Peroxidase Antibody (TPOAb)

Autoantibody targeting TPO; hallmark of Hashimoto’s thyroiditis and early autoimmune hypothyroidism.

Thyroid-Stimulating Immunoglobulin (TSI)

Autoantibody that activates TSH receptors, causing Graves’ hyperthyroidism.

Thyroid-Binding Inhibitory Immunoglobulin (TBII)

Autoantibody that blocks TSH binding, potentially producing hypothyroidism (not routinely tested).

Primary Hypothyroidism

Thyroid gland failure characterized by high TSH with low FT4/FT3.

Secondary (Central) Hypothyroidism

Hypothalamic or pituitary dysfunction with low TSH and low FT4/FT3.

Primary Hyperthyroidism

Excess thyroid hormone production (e.g., Graves’) resulting in low TSH and high FT4/FT3.

Central Hyperthyroidism

TSH-secreting pituitary adenoma yielding high TSH with high T4/T3.

Subclinical Hypothyroidism

Elevated TSH with normal FT4/FT3; mild thyroid failure.

Subclinical Hyperthyroidism

Suppressed TSH with normal T4/T3; early hyperthyroid state.

Hashimoto’s Thyroiditis

Autoimmune destruction of thyroid leading to hypothyroidism; most common U.S. cause.

Graves’ Disease

Autoimmune stimulation of TSH receptors causing diffuse hyperthyroidism and often ophthalmopathy.

Goiter

Enlargement of the thyroid gland; may result from hypo- or hyperthyroidism or iodine deficiency.

Thyroid Nodule

Discrete thyroid mass; usually benign but can represent malignancy.

Thyroid Cancer

Malignant tumor of thyroid presenting with hard, rapidly enlarging nodule and possible lymphadenopathy.

Parathyroid Glands

Four small glands posterior to thyroid that regulate serum calcium via PTH secretion.

Parathyroid Hormone (PTH)

Hormone that increases blood calcium by bone resorption, renal calcium retention, and activating vitamin D.

Intact PTH Test

Laboratory measurement of bioactive PTH to evaluate calcium disorders.

Primary Hyperparathyroidism

Excess PTH secretion (often adenoma) causing hypercalcemia and low phosphate.

Hypoparathyroidism

Deficient PTH production leading to hypocalcemia and hyperphosphatemia.

25-Hydroxyvitamin D

Major circulating inactive vitamin D form; best indicator of vitamin D status.

1,25-Dihydroxyvitamin D (Calcitriol)

Active vitamin D form produced in kidneys under PTH control; enhances intestinal calcium absorption.

DEXA Scan

Dual-energy X-ray absorptiometry measuring bone mineral density at hip and spine.

T-Score

DEXA metric comparing patient BMD to young adult mean; ≤ −2.5 denotes osteoporosis.

Osteopenia

Low bone mass with T-score between −1.0 and −2.5.

Osteoporosis

Skeletal disease of low bone density and fragility fractures; T-score ≤ −2.5.

National Osteoporosis Foundation Guidelines

Recommendations for bone density testing based on age, sex, and risk factors.

Calcium-Phosphate Relationship

Inverse serum relation modulated by PTH: high PTH ↑Ca²⁺, ↓PO₄³⁻; low PTH does opposite.

Osteoporosis Risk Factors

Include female sex, low BMI, aging, smoking, low calcium intake, steroid use, thyroid/parathyroid disease, etc.

Propylthiouracil Monitoring

Checking T3/T4/TSH during therapy; normal values indicate adequate hyperthyroid control.

Hypothyroid Symptoms

Fatigue, cold intolerance, weight gain, dry skin, constipation, bradycardia, depression.

Hyperthyroid Symptoms

Weight loss, heat intolerance, tachycardia, tremor, anxiety, increased stools, goiter.

Graves’ Ophthalmopathy

Autoimmune inflammation causing eye protrusion, redness, diplopia in Graves’ disease.

Chvostek Sign

Facial muscle spasm elicited by tapping facial nerve; indicates hypocalcemia in hypoparathyroidism.

Trousseau Sign

Carpal spasm induced by ischemia (BP cuff); suggests hypocalcemia.

Sodium-Iodide Symporter

Basolateral follicular membrane transporter bringing iodide into thyroid cells.

Organification

Process of iodinating tyrosyl residues on thyroglobulin to form MIT and DIT.

Coupling Reaction

Peroxidase-mediated joining of MIT and DIT to create T3 (MIT+DIT) or T4 (DIT+DIT).

Deiodination

Removal of iodine from MIT/DIT or T4 to recycle iodide and generate active/inactive hormones.

Selenium

Trace element essential for deiodinase activity converting T4 to T3.

Reverse T3/TSH Paradigm

Higher rT3 relative to T3 may mimic hypothyroid symptoms despite normal TSH.

Levothyroxine

Synthetic T4 used to treat hypothyroidism.

Desiccated Natural Thyroid (DNT)

Porcine-derived preparation containing T3 and T4; alternative to synthetic T4 monotherapy.