Thyrotoxicosis: causes
- Mechanism
Too much thyroid hormone speeds up the whole body. It can come from an overactive gland, a leaking inflamed gland, or pills.
What it is
Thyrotoxicosis is the state of excess circulating thyroid hormone; hyperthyroidism is the narrower case in which the gland itself makes too much. The distinction matters because the leaking hormone of thyroiditis or of exogenous intake needs no antithyroid drug. Graves disease, toxic multinodular goitre and toxic adenoma explain most hyperthyroidism.
Function
- Thyroid hormone raises the basal metabolic rate and heat production: weight loss despite good appetite, heat intolerance, sweating.
- It increases beta-adrenergic sensitivity: tachycardia, atrial fibrillation, tremor, lid lag.
- Other effects: diarrhoea, proximal myopathy, osteoporosis, oligomenorrhoea, anxiety.
- Thyroid storm is an acute life-threatening surge, usually in Graves disease after a trigger such as infection or surgery.
- Apathetic hyperthyroidism in older people shows weight loss and atrial fibrillation without the hyperactive picture.
Thyrotoxicosis by source
| Source | Cause | TSH | Iodine uptake |
|---|---|---|---|
| Gland stimulated by antibody | Graves disease | Low | High, diffuse |
| Autonomous nodules | Toxic multinodular goitre, toxic adenoma | Low | High in nodules, rest suppressed |
| Gland stimulated by TSH | Thyrotroph PitNET | Normal or high | High |
| Gland stimulated by hCG | Hydatidiform mole, choriocarcinoma, hyperemesis | Low | High |
| Damaged gland leaking stored hormone | Subacute, painless, postpartum thyroiditis | Low | Low |
| Outside the thyroid | Exogenous hormone, struma ovarii | Low | Low |
Clinical notes
- High-output heart failure and atrial fibrillation are the main cardiac risks.
- Thyrotoxic periodic paralysis: hypokalaemic attacks, mostly in men of Asian origin.
Test yourself
Think of the answer, then tap the question to see it.
What is the difference between thyrotoxicosis and hyperthyroidism?
Thyrotoxicosis is any excess of circulating thyroid hormone; hyperthyroidism is when the thyroid itself overproduces it.
Why is radioactive iodine uptake low in subacute thyroiditis?
The damaged follicles release stored hormone but are not making new hormone, and high hormone suppresses TSH.
Which tumours can cause hyperthyroidism through hCG?
Hydatidiform mole and choriocarcinoma, because hCG weakly activates the TSH receptor.
Elsewhere in DurgiPedia
- Actions of thyroid hormones
- The thyroid axis: reading TSH and free T4
- Hyperthyroidism and hypothyroidism: the signs
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Graves Disease (StatPearls (NCBI Bookshelf), 2025); Thyroid Diseases (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.