Thyroid Disorders for NEET PG — Complete Guide 2026

Version 1.0 — Published March 2026

Thyroid disorders are among the most integrated topics in NEET PG — tested from the medicine angle (clinical features, TFT interpretation), the surgery angle (nodule evaluation, cancer management), the pathology angle (histopathology of cancers), and the pharmacology angle (anti-thyroid drugs, levothyroxine dosing). A single thyroid vignette can test knowledge from four subjects simultaneously. The student who builds a complete mental model of thyroid physiology, dysfunction, and neoplasia captures marks across multiple papers.

This guide covers the full spectrum — from the HPT axis that underpins every TFT question to the Bethesda classification that determines surgical management. Pair this with practice on the Medicine subject hub and cross-reference the high-yield surgery topics for the operative thyroid content.

Thyroid physiology — the HPT axis

The hypothalamic-pituitary-thyroid (HPT) axis is the neuroendocrine feedback loop controlling thyroid hormone production — and understanding this loop is essential for interpreting every thyroid function test question in NEET PG.

The feedback loop:

1. Hypothalamus → releases TRH (thyrotropin-releasing hormone)
2. Anterior pituitary → releases TSH (thyroid-stimulating hormone) in response to TRH
3. Thyroid gland → produces T4 (thyroxine, 90%) and T3 (triiodothyronine, 10%) in response to TSH
4. Peripheral conversion → T4 → T3 by deiodinases (type 1 in liver/kidney, type 2 in brain/pituitary)
5. Negative feedback → T3 and T4 inhibit TSH and TRH release

Key facts for NEET PG:

• T4 is the predominant circulating thyroid hormone (90% of thyroid output) but is relatively inactive
• T3 is the biologically active hormone (3–5x more potent than T4), mostly produced by peripheral conversion
• Reverse T3 (rT3) is an inactive metabolite, elevated in euthyroid sick syndrome
• Thyroid hormone binding: 99.97% bound to proteins (TBG 70%, albumin 20%, transthyretin 10%). Only free T3/T4 is biologically active.
• Wolff-Chaikoff effect: Acute iodine excess → transient inhibition of thyroid hormone synthesis (used therapeutically in thyroid storm with Lugol iodine)
• Jod-Basedow effect: Iodine excess in iodine-deficient glands → hyperthyroidism (opposite of Wolff-Chaikoff)

Thyroid hormone synthesis

The steps of thyroid hormone synthesis are tested as a standalone question — particularly the step each drug blocks:

1. Iodide trapping — NIS (sodium-iodide symporter) on basolateral membrane. Blocked by perchlorate, thiocyanate.
2. Oxidation — iodide → iodine by thyroid peroxidase (TPO). Blocked by thionamides (PTU, methimazole).
3. Organification — iodine + tyrosine on thyroglobulin → MIT, DIT. Blocked by thionamides.
4. Coupling — MIT + DIT → T3; DIT + DIT → T4. Blocked by thionamides.
5. Proteolysis — thyroglobulin breakdown releases T3 and T4 into blood.
6. Peripheral conversion — T4 → T3 by deiodinase. Blocked by PTU (NOT methimazole), propranolol, amiodarone, corticosteroids.

Hypothyroidism

Hypothyroidism is deficient thyroid hormone production — the most common thyroid disorder, affecting 4–10% of the adult population, with a female-to-male ratio of 5–8:1.

Causes

Hashimoto's thyroiditis

Hashimoto's thyroiditis (chronic lymphocytic thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient areas — an autoimmune disorder with lymphocytic infiltration and gradual destruction of thyroid follicles.

Antibodies:

• Anti-TPO (anti-thyroid peroxidase): Most sensitive (95% positive)
• Anti-thyroglobulin: Present in 60–70%

Pathology: Lymphocytic infiltration, germinal centers, Hurthle cell change (eosinophilic metaplasia of follicular cells)

Associations: Type 1 DM, Addison disease, pernicious anemia, celiac disease, Turner syndrome, Down syndrome

NBE point: Hashimoto's increases the risk of thyroid lymphoma (primary thyroid B-cell lymphoma, usually MALT lymphoma or DLBCL). A rapidly enlarging thyroid in a patient with long-standing Hashimoto's should raise suspicion.

Clinical features of hypothyroidism

• General: Fatigue, cold intolerance, weight gain, constipation, dry skin, brittle hair, hoarse voice
• Cardiovascular: Bradycardia, diastolic hypertension, pericardial effusion
• Neurological: Delayed relaxation of deep tendon reflexes (pathognomonic finding), carpal tunnel syndrome, cerebellar ataxia
• Metabolic: Hyperlipidemia (elevated LDL), hyponatremia (impaired free water excretion)
• Reproductive: Menorrhagia (not amenorrhea — a common exam trap), infertility, hyperprolactinemia
• Pediatric: Cretinism (intellectual disability, short stature, coarse features, delayed bone age, umbilical hernia, macroglossia)

Myxedema coma

Myxedema coma is the most severe and life-threatening manifestation of hypothyroidism — a medical emergency with 30–60% mortality.

Triggers: Infection, cold exposure, sedatives, surgery, trauma in a patient with untreated or undertreated hypothyroidism.

Clinical features: Hypothermia (<35 degrees C), altered consciousness progressing to coma, bradycardia, hypoventilation with CO2 retention, hyponatremia, hypoglycemia.

Management:

1. IV levothyroxine loading dose: 200–400 mcg bolus, then 50–100 mcg/day
2. IV hydrocortisone: 100 mg bolus, then 50 mg q8h (MUST give steroids before or with thyroid hormone — prevents precipitating adrenal crisis in concurrent adrenal insufficiency)
3. Passive rewarming (NOT active — causes vasodilation and circulatory collapse)
4. Supportive: ventilation, fluid restriction for hyponatremia, treat precipitating cause

Hyperthyroidism

Hyperthyroidism is excess thyroid hormone production or release — presenting with a hypermetabolic state of heat intolerance, weight loss, tremor, and tachycardia.

Graves disease

Graves disease is the most common cause of hyperthyroidism (60–80% of cases) — an autoimmune disorder caused by thyroid-stimulating immunoglobulins (TSI) that activate the TSH receptor.

Clinical triad (unique to Graves, NOT seen in other causes of hyperthyroidism):

1. Diffuse goiter — smooth, non-tender, with thyroid bruit (increased vascularity)
2. Ophthalmopathy — proptosis, lid retraction, lid lag, periorbital edema, diplopia (extraocular muscle infiltration). Present in 30–50% of patients.
3. Dermopathy — pretibial myxedema (non-pitting edema over shins with peau d'orange texture). Present in <5% of patients.

Antibodies: TSI (thyroid-stimulating immunoglobulin) — pathogenic; anti-TPO often positive as well.

Radioiodine uptake scan: Diffusely increased uptake (differentiates from subacute thyroiditis where uptake is LOW).

Toxic multinodular goiter and toxic adenoma

Thyroid storm

Thyroid storm is a life-threatening exacerbation of hyperthyroidism — mortality 10–30% even with treatment.

Triggers: Surgery, infection, RAI therapy, iodinated contrast, trauma, DKA in a patient with uncontrolled hyperthyroidism.

Clinical features: Fever >40 degrees C, tachycardia >140 bpm, altered mental status (agitation → delirium → coma), cardiac failure, GI dysfunction (nausea, vomiting, diarrhea, jaundice).

Management (order matters):

1. PTU (NOT methimazole) — blocks new hormone synthesis AND peripheral T4→T3 conversion. Loading dose 500–1000 mg, then 250 mg q4h.
2. Lugol iodine or SSKI — given 1 HOUR after PTU (Wolff-Chaikoff effect blocks hormone release). If given before PTU, iodine provides substrate for new hormone synthesis.
3. Beta-blocker — propranolol (also blocks T4→T3 conversion). Esmolol IV if oral not possible.
4. Hydrocortisone — 100 mg IV q8h (blocks T4→T3 conversion + treats relative adrenal insufficiency)
5. Supportive — cooling, fluids, treat precipitating cause

Thyroid function test interpretation

TFT interpretation is a systematic approach to reading TSH and free T4/T3 levels — tested in every NEET PG paper, often as a table-matching question.

Subclinical hypothyroidism treatment criteria:

• TSH >10 mU/L → treat with levothyroxine
• TSH 4.5–10 mU/L → treat if: anti-TPO positive, symptoms present, pregnancy, infertility, or cardiovascular risk factors

Euthyroid sick syndrome (non-thyroidal illness):

• Seen in critically ill patients
• Low T3, elevated rT3 (reverse T3), variable TSH (usually low-normal)
• Do NOT treat — resolves with recovery from underlying illness

Thyroid nodule evaluation

Thyroid nodule evaluation is the systematic assessment of thyroid nodules to exclude malignancy — 4–7% of the general population has palpable nodules, and 5–15% of these are malignant.

Initial workup:

1. TSH — if low (suggesting a hot/functioning nodule), proceed to radioiodine scan. Hot nodules are almost never malignant.
2. Ultrasound — characterize the nodule (solid vs cystic, calcifications, margins, vascularity)
3. FNAC — the investigation of choice for thyroid nodules. Indicated for all solid nodules >1 cm and nodules with suspicious ultrasound features.

Bethesda system for reporting thyroid FNAC

Key NBE point: FNAC cannot distinguish follicular adenoma from follicular carcinoma — the distinction requires demonstrating capsular or vascular invasion on histopathology, which needs surgical excision (lobectomy). This is why Bethesda IV mandates diagnostic lobectomy.

Thyroid cancers

Thyroid cancer is the most common endocrine malignancy — classified by the cell of origin into differentiated (papillary, follicular), medullary (C cells), and anaplastic.

Papillary thyroid carcinoma — the most tested thyroid cancer

• Most common thyroid cancer and most common radiation-induced thyroid cancer
• Histopathology hallmarks: Orphan Annie eye nuclei (clear, ground-glass nuclei), psammoma bodies (concentric calcifications), nuclear grooves and pseudoinclusions
• Spreads via lymphatics — cervical lymph node metastases are common but do NOT worsen prognosis significantly (unlike most cancers)
• Treatment: Total thyroidectomy + radioiodine ablation + TSH suppression with levothyroxine
• Follow-up: Serum thyroglobulin (tumor marker for recurrence after thyroidectomy — should be undetectable)

Medullary thyroid carcinoma — the MEN-associated cancer

• Arises from parafollicular C cells (neural crest origin)
• Produces calcitonin (tumor marker) and CEA
• Amyloid deposits in stroma (Congo red positive — apple-green birefringence under polarized light)
• NOT RAI-avid — radioiodine is useless (C cells do not take up iodine)
• Familial forms: MEN 2A (MTC + pheo + hyperPTH), MEN 2B (MTC + pheo + mucosal neuromas + marfanoid habitus), familial MTC (MTC only)
• RET proto-oncogene mutation — screen family members. Prophylactic thyroidectomy for carriers.

Thyroid in pregnancy

Thyroid management in pregnancy is a high-yield area tested from both the medicine and obstetrics perspectives — with specific drug restrictions and screening criteria that NEET PG tests directly.

Physiological changes in pregnancy:

• HCG stimulates the TSH receptor (structural similarity) → transient TSH suppression in first trimester (gestational thyrotoxicosis)
• TBG increases (estrogen effect) → total T4 increases, but free T4 remains normal
• TSH reference ranges change: first trimester upper limit is 4.0 mU/L (lower than non-pregnant)

Hypothyroidism in pregnancy:

• Untreated maternal hypothyroidism → cretinism, preterm delivery, preeclampsia, placental abruption
• Treatment: levothyroxine (increase dose by 30–50% as soon as pregnancy is confirmed)
• Target TSH: <2.5 mU/L in first trimester, <3.0 mU/L in second and third trimesters

Hyperthyroidism in pregnancy:

• First trimester: PTU (propylthiouracil) is preferred — methimazole causes aplasia cutis and choanal atresia
• Second and third trimester: Switch to methimazole (PTU carries hepatotoxicity risk)
• Radioiodine is ABSOLUTELY CONTRAINDICATED in pregnancy (destroys fetal thyroid)
• Beta-blockers: propranolol can be used short-term for symptomatic control

Anti-thyroid drugs — PTU vs methimazole

Anti-thyroid drugs (thionamides) are the pharmacological agents used to treat hyperthyroidism — and the PTU vs methimazole comparison is one of the most predictable NEET PG pharmacology questions.

Agranulocytosis — the critical ADR:

• Occurs in 0.1–0.5% of patients on either drug
• Presents as sore throat, fever, mouth ulcers in a patient on anti-thyroid medication
• Immediate action: Stop the drug, check WBC with differential
• Do NOT switch to the other thionamide (cross-reactivity exists)
• Alternative treatment: radioiodine ablation or surgery

Sources and references

1. Harrison's Principles of Internal Medicine, 21st Edition (Loscalzo et al., 2022) — Chapters on thyroid physiology and thyroid disorders.
2. Williams Textbook of Endocrinology, 14th Edition (Melmed et al., 2020) — detailed thyroid hormone synthesis, regulation, and cancer.
3. SRB's Manual of Surgery, 7th Edition (Bhat, 2023) — thyroid surgery indications, techniques, and complications.
4. 2015 ATA Management Guidelines for Thyroid Nodules and Differentiated Thyroid Cancer (Haugen et al., Thyroid 2016) — nodule evaluation and Bethesda system.
5. 2023 European Thyroid Association Guidelines on Thyroid Disorders in Pregnancy — TSH targets and drug safety in pregnancy.

Frequently asked questions

How many thyroid questions appear in NEET PG?

Thyroid disorders contribute 3-5 direct questions per NEET PG paper across medicine, surgery, pathology, and pharmacology. TFT interpretation, Graves disease features, thyroid cancer classification, and anti-thyroid drug side effects are the four most frequently tested areas based on 2019-2025 analysis.

How do I interpret thyroid function tests in NEET PG?

Low TSH with high T3/T4 indicates primary hyperthyroidism (Graves, toxic nodule). High TSH with low T3/T4 indicates primary hypothyroidism (Hashimoto's). Low TSH with low T3/T4 indicates secondary (pituitary) hypothyroidism. High TSH with normal T3/T4 is subclinical hypothyroidism. Always start with TSH — it is the most sensitive screening test.

What is the most common cause of hypothyroidism in India?

Iodine deficiency remains the most common cause of hypothyroidism worldwide and in iodine-deficient regions of India. In iodine-sufficient areas, Hashimoto's thyroiditis (chronic autoimmune thyroiditis) is the most common cause. Anti-TPO antibodies are the most sensitive marker for Hashimoto's.

What is the most common thyroid cancer?

Papillary thyroid carcinoma accounts for 80-85% of all thyroid cancers. It has the best prognosis, spreads via lymphatics (cervical lymph nodes), and shows Orphan Annie eye nuclei and psammoma bodies on histopathology. The typical patient is a young female with a cold nodule on thyroid scan.

When should PTU be preferred over methimazole?

PTU is preferred over methimazole only in the first trimester of pregnancy (methimazole is teratogenic — aplasia cutis, choanal atresia) and in thyroid storm (PTU inhibits peripheral T4-to-T3 conversion in addition to blocking thyroid hormone synthesis). In all other situations, methimazole is preferred due to once-daily dosing and fewer side effects.

What is the Bethesda system for thyroid nodule classification?

The Bethesda system classifies thyroid FNAC results into 6 categories: I (non-diagnostic, repeat FNAC), II (benign, follow-up), III (atypia of undetermined significance, repeat FNAC or molecular testing), IV (follicular neoplasm, diagnostic lobectomy), V (suspicious for malignancy, near-total thyroidectomy), VI (malignant, total thyroidectomy).

How do I differentiate Graves disease from toxic multinodular goiter?

Graves disease presents in younger women (20-40 years) with diffuse goiter, exophthalmos, pretibial myxedema, and thyroid-stimulating immunoglobulin (TSI) positivity. Radioiodine uptake is diffusely increased. Toxic MNG presents in older patients (above 50) with nodular goiter, no eye signs, and patchy radioiodine uptake.

What is myxedema coma and how is it managed?

Myxedema coma is a life-threatening decompensation of severe hypothyroidism presenting with altered consciousness, hypothermia, bradycardia, hypoventilation, and hyponatremia. Treatment is IV levothyroxine loading dose (200-400 mcg), IV hydrocortisone (to prevent adrenal crisis before thyroid replacement), passive rewarming, and supportive care. Mortality is 30-60%.

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Written by: NEETPGAI Editorial Team Reviewed by: Pending SME Review Last reviewed: March 2026

This article is reviewed by qualified medical professionals for clinical accuracy and exam relevance. For corrections or updates, contact the editorial team.