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Thyroid 101

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I now have a comprehensive picture from multiple authoritative textbooks. Let me compile the full Thyroid 101 overview.

Thyroid 101 - A Complete Overview

1. Anatomy

The thyroid gland is a butterfly-shaped endocrine organ that sits in the anterior neck, just inferior to the larynx and straddling the trachea. It has two lobes connected by a narrow isthmus. Key anatomical points:
  • Weight: ~20-30 g in an adult
  • Blood supply: highly vascular - blood flow is approximately 5x the gland's weight per minute, comparable only to the adrenal cortex
  • Microscopic structure: thousands of closed follicles (100-300 micrometers in diameter) filled with colloid and lined by cuboidal follicular (thyrocyte) epithelium
  • Colloid contains thyroglobulin (Tg), the major secretory glycoprotein and hormone precursor
  • C cells (parafollicular cells) scattered between follicles - they secrete calcitonin, which regulates plasma calcium
Guyton and Hall Textbook of Medical Physiology

2. Hormone Synthesis - Step by Step

The HPT axis (Hypothalamus - Pituitary - Thyroid) controls the whole process:
Thyroid hormone synthesis and HPT axis regulation - showing TRH→TSH→thyroid feedback loop with NIS, thyroglobulin iodination, and T3/T4 release
Regulation of thyroid hormone synthesis: TRH stimulates TSH; TSH acts on follicular cells via cAMP to drive NIS-mediated iodide uptake, thyroglobulin synthesis, and iodination. T4 and T3 feed back negatively to both hypothalamus and pituitary. (Harrison's 22E)

Step-by-step synthesis:

StepWhat Happens
1. Iodide trappingSodium-iodide symporter (NIS) on the basolateral membrane co-transports 2 Na⁺ + 1 I⁻ into the follicular cell (secondary active transport driven by Na/K-ATPase). Concentrates I⁻ 20-40x above plasma
2. Iodide effluxPendrin (Cl⁻/I⁻ exchanger) on the apical membrane transports I⁻ into the colloid. Mutation → Pendred syndrome (goiter + deafness)
3. OrganificationThyroid peroxidase (TPO) at the apical membrane oxidizes I⁻ to reactive iodine and attaches it to tyrosine residues on thyroglobulin → forms monoiodotyrosine (MIT) and diiodotyrosine (DIT)
4. CouplingTPO couples MIT + DIT → T3 (triiodothyronine); DIT + DIT → T4 (thyroxine). The hormones remain bound to thyroglobulin in the colloid
5. StorageColloid acts as a reservoir - humans can survive 2 months on zero dietary iodine before hormone levels fall
6. SecretionOn TSH stimulation, follicular cells endocytose colloid → lysosomes hydrolyze thyroglobulin → free T4 and T3 released into capillaries
7. RecyclingMIT and DIT not secreted - deiodinase recovers the iodine for reuse. This provides ~2x more iodine than NIS intake
Ganong's Review of Medical Physiology, 26E; Harrison's 22E
Daily secretion: ~80 µg T4, ~4 µg T3, ~2 µg reverse T3 (rT3)

3. Transport in Blood

Once in the circulation, thyroid hormones are lipophilic and bind heavily to plasma proteins:
ProteinT4 bound (%)T3 bound (%)
Thyroxine-binding globulin (TBG)67%46%
Transthyretin (TBPA)20%1%
Albumin13%53%
  • T4: 99.98% bound; free T4 only ~2 ng/dL; half-life ~6-7 days
  • T3: 99.8% bound; free T3 ~0.3 ng/dL; shorter half-life, more rapid action
  • Only free hormone is biologically active and feeds back to the pituitary
Key modifiers of TBG levels:
  • TBG ↑: estrogens, pregnancy, OCP → total T4 rises but patient is euthyroid (free T4 normal)
  • TBG ↓: glucocorticoids, androgens, nephrotic syndrome
Ganong's Review of Medical Physiology, 26E

4. Peripheral Metabolism: T4 → T3

T4 is a prohormone. The majority of circulating T3 (the active form) comes from peripheral deiodination of T4 by deiodinase enzymes in liver, kidney, and other tissues:
  • Type 1 & 2 deiodinase: remove iodine from the outer ring → produce active T3
  • Type 3 deiodinase: remove iodine from the inner ring → produce reverse T3 (rT3), which is inactive
  • T3 is ~3-4x more potent than T4 at the nuclear receptor

5. Mechanism of Action

Thyroid hormones act primarily on nuclear receptors (TR-α and TR-β):
  • Free T3/T4 diffuses across the cell membrane
  • T3 binds thyroid receptor → receptor-hormone complex binds thyroid response elements (TREs) on DNA → alters gene transcription
Major physiological effects:
  • Increase basal metabolic rate (O₂ consumption, heat generation)
  • Stimulate protein synthesis and carbohydrate/fat metabolism
  • Essential for normal growth, brain development (especially fetal/neonatal)
  • Increase sensitivity to catecholamines (↑ heart rate, cardiac output)
  • Required for normal GI motility
  • Regulate bone turnover

6. The HPT Feedback Axis

Hypothalamus
    ↓ TRH (+)
Anterior Pituitary
    ↓ TSH (+)
Thyroid Gland
    ↓ T4 / T3 (+)
Peripheral Tissues
    ↕
Free T3/T4 — negative feedback on both hypothalamus & pituitary
  • TSH (thyroid-stimulating hormone) is the master regulator - even tiny changes in free T4/T3 cause large inverse TSH changes
  • TSH acts via Gs-protein → cAMP → drives all steps of hormone synthesis and thyroid growth

7. Clinical Conditions at a Glance

Hypothyroidism

Cause: Most commonly Hashimoto's thyroiditis (autoimmune destruction); also iodine deficiency, post-thyroidectomy, post-radioiodine
TSH/hormone pattern: TSH ↑, free T4 ↓
Symptoms (think: everything slowed down):
  • Fatigue, cold intolerance, weight gain, constipation
  • Bradycardia, dry skin, hair loss, myxedema (non-pitting edema)
  • Delayed relaxation of deep tendon reflexes
  • Elevated TSH is the most sensitive screening test
Treatment: Levothyroxine (LT4) - typical dose 1.6 µg/kg/day. Goal: TSH in lower half of normal range. Check TSH 6-8 weeks after dose changes.
Harrison's Principles of Internal Medicine 22E

Hyperthyroidism / Thyrotoxicosis

Cause: Most commonly Graves' disease (TSH-receptor stimulating antibodies); also toxic multinodular goiter, toxic adenoma
TSH/hormone pattern: TSH ↓, free T4/T3 ↑
Symptoms (think: everything sped up):
  • Palpitations, tachycardia, atrial fibrillation
  • Heat intolerance, sweating, weight loss despite increased appetite
  • Tremor, anxiety, irritability
  • Diarrhea/hyperdefecation, increased bowel motility
  • Systolic hypertension, increased cardiac output, reduced peripheral vascular resistance
Cardiovascular note: Thyrotoxicosis carries the highest morbidity risk via AF, cardiac hypertrophy, and heart failure. The risk of cardiovascular death is higher than in euthyroid individuals. Beta-blockers (propranolol) help control adrenergic symptoms even though catecholamine levels are actually normal.
Treatment: Beta-blockers (symptom control), thionamides (PTU, methimazole - block TPO), radioiodine ablation, thyroidectomy
Tietz Textbook of Laboratory Medicine, 7E; Harrison's 22E

Graves' Disease - Special Features

  • Diffuse goiter (smooth, enlarged thyroid)
  • Ophthalmopathy (proptosis, lid lag, periorbital edema) - from retroorbital inflammation
  • Pretibial myxedema (dermopathy) - orange-peel skin on shins
  • TSH-receptor antibodies (TRAb/TSI) are diagnostic

8. Lab Interpretation Quick Reference

ConditionTSHFree T4Free T3
Primary hypothyroidism↑↑↓↓
Subclinical hypothyroidism↑NormalNormal
Hyperthyroidism↓↓↑↑
Subclinical hyperthyroidism↓NormalNormal
Secondary hypothyroidism (pituitary)↓ or normal↓↓
Non-thyroidal illness ("sick euthyroid")Variable↓ T3, ↑ rT3↓
High TBG (e.g., pregnancy)NormalNormalNormal
TSH is the single best first-line test for thyroid function in outpatient screening. Free T4 adds information once TSH is abnormal.

9. Iodine - The Essential Substrate

  • Minimum dietary requirement: ~50 mg/year (~1 mg/week)
  • Normal uptake: ~10-25% of ingested radioiodine absorbed by thyroid in 24h; up to 70-90% in Graves' disease
  • Both deficiency AND excess inhibit thyroid function (Wolff-Chaikoff effect for excess)
  • Worldwide, iodine deficiency affects ~2 billion people and is the leading preventable cause of intellectual disability (cretinism in severe neonatal deficiency)
  • NIS expression in the thyroid (but not most other tissues) is the basis for radioiodine scanning and treatment of thyroid cancer
Harrison's 22E; Ganong's 26E; Guyton & Hall
Sources consulted:
  • Harrison's Principles of Internal Medicine 22E (2025)
  • Ganong's Review of Medical Physiology, 26E
  • Guyton and Hall Textbook of Medical Physiology
  • Tietz Textbook of Laboratory Medicine, 7E
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