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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Thyroid hormones

Synthetic T4 (levothyroxine) and T3 (liothyronine) that replace the hormone an underactive thyroid no longer makes.

Updated 2026-11-01

## What they are

Replacement for hypothyroidism of any cause and suppression of TSH after thyroid cancer surgery. Levothyroxine (T4) is the standard; tissues turn it into T3. Liothyronine (T3) acts faster and shorter and has special uses.

## Mechanism of action

T3 enters cells and binds thyroid hormone receptors (TRα and TRβ), nuclear receptors that sit on DNA with RXR. Hormone binding releases corepressors and recruits coactivators, changing transcription of genes for metabolism, heat production, heart rate and growth. T4 is mostly a prohormone: deiodinases in tissues remove one iodine to make T3.

Target: Thyroid hormone receptors TRα and TRβ (nuclear receptors)

## Key points

- TSH is the main test to adjust replacement in primary hypothyroidism; free T4 is used in central (pituitary) hypothyroidism.
- Uncorrected adrenal insufficiency must be treated first: thyroid hormone can trigger an adrenal crisis.
- Start low in the elderly and in coronary disease: excess hormone can cause angina and arrhythmias.
- Not for weight loss (boxed warning): in people with normal thyroid function it does not help and large amounts are toxic.

## Drugs in this class

- [**Levothyroxine** Synthetic thyroid hormone (T4) taken daily to replace what an underactive thyroid no longer makes. Adjusted with blood… Thyroid hormone receptor β (and α)](https://durgical.com/en/pharmacology/levothyroxine/)
- [**Liothyronine** Synthetic T3, the active thyroid hormone. It acts within hours and wears off faster than levothyroxine. Thyroid hormone receptor β (and α)](https://durgical.com/en/pharmacology/liothyronine/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which receptor mediates the effects of thyroid hormone, and where is it?

Thyroid hormone receptors (TRα, TRβ), nuclear receptors bound to DNA with RXR that change gene transcription when T3 binds.

## Why must adrenal insufficiency be corrected before starting thyroid hormone?

Thyroid hormone speeds cortisol metabolism and raises its demand, which can precipitate an adrenal crisis.

## Which test adjusts levothyroxine in central (pituitary) hypothyroidism?

Free T4, because the pituitary cannot make a reliable TSH response.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - LEVOTHYROXINE SODIUM tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0fa2e2f5-401b-45c1-b6a5-d02f6faf509d) (DailyMed (U.S. National Library of Medicine), 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 38 (McGraw Hill, 2025); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/) (World Health Organization, 2025).

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