Thyroid Disorders — Hypothyroidism vs Hyperthyroidism Complete Guide

Creator: Progress Note
Published: 2026-07-22
Keywords: hypothyroidism vs hyperthyroidism, thyroid function tests interpretation, Graves disease vs Hashimoto, thyroid storm treatment, TSH T4 interpretation

The 60-second version

High TSH + low free T4 = primary hypothyroidism (treat with levothyroxine)
Low TSH + high free T4 = primary hyperthyroidism
Graves': TRAb, diffuse uptake, exophthalmos — treat with methimazole
Thyroid storm: PTU → SSKI (after PTU) → beta-blockers → steroids
Sick euthyroid: do not treat — TFTs normalize with underlying illness recovery

TFT Interpretation

Primary hypothyroidism (most common — Hashimoto's): TSH high, free T4 low. Primary hyperthyroidism (Graves', toxic nodule): TSH low, free T4 high. Secondary hypothyroidism (pituitary failure): TSH low (or inappropriately normal), free T4 low. Subclinical hypothyroidism: TSH high, free T4 normal. Sick euthyroid (non-thyroidal illness): low T3, low/normal T4, TSH variable — do not treat, TFTs normalize with recovery.

Hypothyroidism: Causes and Treatment

Most common cause worldwide: iodine deficiency. Most common cause in developed countries: Hashimoto's thyroiditis (anti-TPO, anti-thyroglobulin antibodies). Presentation: fatigue, weight gain, cold intolerance, constipation, dry skin, hair loss, bradycardia, delayed relaxation of reflexes. Treatment: levothyroxine (T4). Monitor TSH every 6–8 weeks until stable. Target TSH 0.5–2.5 mIU/L.

Hyperthyroidism: Graves vs Toxic Nodule

Graves' disease (most common): TSH-receptor autoantibodies (TRAb) stimulate thyroid continuously. Features: diffuse goiter, exophthalmos (proptosis), pretibial myxedema (rare but specific), high radioactive iodine uptake (diffuse). Toxic adenoma/multinodular goiter: focal uptake on radioiodine scan ('hot nodule'). Treatment: beta-blockers first (symptom control), then methimazole (PTU in pregnancy first trimester), radioiodine ablation, or surgery.

Thyroid Storm vs Myxedema Coma

Thyroid storm: extreme hyperthyroidism (fever >38.5°C, tachycardia, agitation, delirium), precipitated by surgery/illness/iodine load. Treat: PTU (blocks synthesis + peripheral conversion), SSKI/Lugol's (blocks release, give after PTU), beta-blockers, steroids (dexamethasone blocks T4→T3). Myxedema coma: extreme hypothyroidism (hypothermia, bradycardia, altered mentation, CO2 retention). Treat: IV T4 and/or T3, hydrocortisone (coexisting adrenal insufficiency), supportive care.

Test yourself

Quick check — 3 questions

1. A 35-year-old woman has TSH 0.02 mIU/L, free T4 elevated, diffuse goiter, and exophthalmos. Most likely diagnosis?

2. A pregnant woman at 8 weeks has hyperthyroidism. The preferred initial antithyroid medication is:

3. A patient presents with fever 39.8°C, HR 148, confusion, and a known history of Graves' disease. The first antithyroid medication to give is:

Frequently asked questions

Why is PTU preferred over methimazole in the first trimester of pregnancy?

Methimazole is associated with aplasia cutis and choanal/esophageal atresia (methimazole embryopathy) when used in the first trimester. PTU (propylthiouracil) is preferred in first-trimester hyperthyroidism. In the second and third trimesters, methimazole is preferred because PTU carries a risk of severe hepatotoxicity. Both cross the placenta and can cause fetal hypothyroidism.

What is the significance of exophthalmos in Graves' disease?

Proptosis (exophthalmos) in Graves' disease is caused by infiltration of the retro-orbital space with glycosaminoglycans and lymphocytes driven by TSH-receptor antibodies (TRAb) cross-reacting with orbital fibroblasts — not by excess thyroid hormone itself. This is why exophthalmos does NOT reliably improve after thyroidectomy or radioiodine (which only treats the thyroid) and may worsen after RAI. Treatment: selenium, teprotumumab, orbital decompression for severe cases.

What precipitates thyroid storm?

Thyroid storm is precipitated by physiological stress in an uncontrolled or undertreated hyperthyroid patient. Common precipitants: surgery (especially thyroid surgery), acute illness (infection, MI, trauma), iodine load (IV contrast, amiodarone, iodine-containing medications), and abrupt withdrawal of antithyroid medications. Management sequence: PTU → SSKI (wait 1 hour after PTU) → propranolol → dexamethasone.

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