Endocrine · Topic 8 of 13

Hypothyroidism, Hyperthyroidism & Thyroid Tests

Endocrine · thyroid function

Hypothyroidism, Hyperthyroidism & Thyroid Tests

TSH first, then free T4 to classify. In the critically ill, abnormal tests are usually nonthyroidal illness — don't order or treat them reflexively.

Key numbers

TSH is the best first testTSH first

Avoid thyroid tests in critical illnessTSH usually < 10

Levothyroxine 1.6 mcg/kg/day full replacement1.6 mcg/kg/day

Treat subclinical hypothyroidism when TSH over 10TSH > 10 mIU/L

01 · The ruleTSH is the best first test

Serum TSH is the single best test of thyroid function; a normal TSH excludes primary hypo- and hyperthyroidism, except in central (pituitary or hypothalamic) disease or rare pituitary hormone resistance.

02 · The ruleAvoid thyroid tests in critical illness

In euthyroid sick syndrome T3 (then T4) falls while TSH stays low, normal or slightly high (usually < 10 mIU/L). Test critically ill patients only when thyroid disease is strongly suspected; never give thyroid hormone.

03 · GiveLevothyroxine 1.6 mcg/kg/day full replacement

Full replacement is levothyroxine 1.6 mcg/kg/day; start 25 mcg daily in older patients or those with heart disease and titrate by 12.5–25 mcg. Recheck TSH 6–8 weeks after any dose change; target a mid-normal TSH.

Diagnosis & monitoring

Work-up

Classify by TSH and free T4 together must knowPrimary hypothyroidism: high TSH, low free T4. Subclinical: high TSH, normal free T4. Hyperthyroidism: low TSH, high free T4 or T3. Central hypothyroidism: low or normal TSH, low free T4.

On amiodarone, check TSH every 6 monthsCheck TSH and free T4 before starting amiodarone and about every 6 months during therapy.

Fever or sore throat on thionamide: check CBCThionamides cause agranulocytosis in about 0.1% and PTU can cause liver failure; check a CBC for fever or sore throat, stop the drug if agranulocytosis occurs and never switch to the other thionamide.

Management

Orders

Amiodarone hypothyroidism: add levothyroxineAmiodarone is about 37% iodine by weight and causes thyroid dysfunction in about 15–20% of patients; amiodarone-induced hypothyroidism is treated with levothyroxine, usually without stopping amiodarone.

Methimazole first; beta-blocker for symptomsMethimazole (5–20 mg 1–3 times daily initially) is the preferred thionamide, with a beta-blocker for tachycardia and tremor; PTU is reserved for the first trimester and thyroid storm.

Red flags & don'ts

Avoid harm

Dopamine and glucocorticoids lower TSH interactionDopamine and glucocorticoids suppress pituitary TSH, while amiodarone and iodinated contrast block T4-to-T3 conversion — common causes of misleading thyroid tests in hospitalized patients.

The rules

Know these

Treat subclinical hypothyroidism when TSH over 10Subclinical hypothyroidism with TSH > 10 mIU/L usually progresses and should be treated with levothyroxine even without symptoms; for TSH 4.5–10, consider a trial only if symptomatic, otherwise recheck yearly.

Subclinical hyperthyroidism: treat TSH below 0.1Endogenous subclinical hyperthyroidism with TSH < 0.1 mIU/L raises the risk of atrial fibrillation, bone loss and fractures and warrants treatment; if it is from levothyroxine, lower the dose.

Central hypothyroidism: replace cortisol firstIn central hypothyroidism dose levothyroxine to a mid-normal free T4 (TSH is useless), and confirm adequate cortisol or start hydrocortisone first, because levothyroxine can precipitate adrenal crisis.

Amiodarone thyrotoxicosis: type 1 or type 2?

Commonly confused

Underlying gland	Nodular goiter or latent Graves
Mechanism	Iodine-driven excess hormone synthesis
First-line treatment	Thionamide (methimazole)

Commonly confused

Tell apart

Radioiodine uptake separates Graves from thyroiditisRadioactive iodine uptake is high and diffuse in Graves disease, focal in toxic nodules, and low in thyroiditis, iodine excess and exogenous thyroid hormone (thyroglobulin is low only in factitious use).

Teach & follow up

Patient education

Take levothyroxine on an empty stomachTake levothyroxine once daily 30–60 minutes before breakfast and at least 4 hours apart from calcium, iron, phosphate binders and bile acid sequestrants; enzyme inducers (rifampin, carbamazepine, phenytoin) raise the dose needed.

More detail

Reference

Show 5 reference facts

In the first 3 months of amiodarone, free T4 rises, T3 falls and TSH may rise transiently 20–50% in euthyroid patients; these expected changes are not disease.

During recovery from nonthyroidal illness, TSH can rise transiently above normal; repeat tests after recovery before diagnosing hypothyroidism.

Total T4 changes with binding proteins (estrogen raises TBG and total T4) while free T4 stays normal; serum T3 is insensitive for hypothyroidism.

Radioiodine (I-131) is the most common US treatment for Graves disease and toxic nodular goiter; most patients become hypothyroid, and it is contraindicated in pregnancy and lactation.

Lithium, interferon-alfa, checkpoint inhibitors and some tyrosine kinase inhibitors cause hypothyroidism; checkpoint inhibitors and alemtuzumab can also cause thyroiditis with transient thyrotoxicosis.

Pearl

Read the TSH first; it moves before free T4 does — except when the pituitary is the problem or the patient is in the ICU.

Sources · Merck Manual Professional — Overview of Thyroid Function (table: Results of Thyroid Function Tests in Various Clinical Situations) · Merck Manual Professional — Hypothyroidism, 2026 · Merck Manual Professional — Euthyroid Sick Syndrome, Jan 2026 · Synthroid (levothyroxine sodium) prescribing information — DailyMed · Amiodarone and thyroid dysfunction (review). Acta Clin Croat 2022;61(2) · Merck Manual Professional — Hyperthyroidism, Jan 2026 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).