What’s the difference between hyperthyroidism, thyrotoxicosis, and thyroid storm?
Hyperthyroidism refers to conditions in which the production of thyroid hormone is increased. Thyrotoxicosis is any state where the concentration of thyroid hormone is increased in the circulation, which includes overproduction, release from an injured gland, or exogenous hormone. Thyroid storm is a life-threatening decompensation of thyrotoxicosis, with extremes in clinical symptoms including fever, marked tachycardia, CNS and GI dysfunction, with decompensation of one organ system, such as shock or heart failure. 1
When should I think about hyperthyroidism in my patients?
Have a low suspicion to screen for hyperthyroidism in patients who have symptoms that cannot be explained by other causes. Fatigue and generalized weakness is very common, along with weight loss and decreased appetite. Neuropsychiatric complaints include anxiety, tremor, insomnia, and emotional lability. Cardiopulmonary symptoms are very common and include palpitations, tachycardia, atrial fibrillation, and dyspnea on exertion. In these patients, examining their thyroid for enlargement or nodules is helpful, and a screening TSH should be part of their workup. 1
When should I think about thyroid storm?
Thyroid storm is a clinical diagnosis. Pay attention to certain features in the history, vitals, and exam:
- Underlying hyperthyroidism – Most common in patients with a history of Graves’ disease, and occasionally in toxic multinodular goiter and toxic adenoma
- Fever – Thyroid storm patients have lost the ability to vasodilate and thermoregulate. Temperature can exceed 104-106 F.
- Altered mental status – A key to diagnosis. This ranges from restlessness to delirium, seizures, or coma.
- Suspected precipitating event – Common causes include infection, trauma, surgery, and excessive diuresis.
Other signs and symptoms can include marked tachycardia (can be >140 bpm), congestive heart failure, and GI symptoms like nausea, vomiting, and abdominal pain.
Hyperthyroidism is especially difficult to diagnose in the elderly, who will often not show overt physical signs. Clues to diagnosis include supraventricular arrhythmias, heart failure, inappropriately youthful skin.
Thyroid storm has a high mortality rate. Bottom line: if you suspect it, treat it. 2
What labs are useful in diagnosing hyperthyroidism? Is TSH enough or should I send free T4 and T3?
Serum TSH measurement has the highest sensitivity and specificity of any single blood test used in the evaluation of suspected hyperthyroidism and should be used as an initial screening test. A normal TSH essentially excludes hyperthyroidism (except in the rare circumstance of a TSH-producing pituitary adenoma or thyroid hormone resistance). Measurement of thyroid hormone levels (free T4 and free T3) is subsequently required for definitive diagnosis.
Other lab abnormalities seen in hyperthyroidism include anemia, thrombocytopenia, low creatinine (cannot convert creatine to creatinine), hyperglycemia, and hypercalcemia. 3,4
What is the differential diagnosis when TSH is low?
| >TSH | >Free T4 | >Free T3 | >Condition |
| Normal | Normal | Normal | None |
| Low | High | High | Hyperthyroidism |
| Low | Normal | Normal | Subclinical hyperthyroidism |
| Low | Normal | High | T3 toxicosis |
| Low | High | Normal | Thyroiditis, T4 ingestion, hyperthyroidism in chronic illness |
| Low | Low | Low | Euthyroid sick syndrome, central hypothyroidism 1 |
How do I diagnose hyperthyroidism in pregnancy?
TSH levels in the first half of pregnancy may normally be lower than the non-pregnant population, due to stimulation of the thyroid by serum bHCG. In the second half of pregnancy, TSH levels will return back to normal. Due to variations in thyroxine-binding globulin (TBG), free T3 and free T4 levels are also trimester-specific and each laboratory has their own reference values.
According to 2011 Guidelines from American Thyroid Association, normal TSH levels in pregnancy are:
- First trimester: 0.1-2.5 mIU/L
- Second trimester: 0.2-3.0 mIU/L
- Third trimester: 0.3-3.0 mIU/L 5
Is measuring TSH useful in undifferentiated Emergency Department patients with palpitations or atrial fibrillation?
So far there have been no studies performed on the prevalence of hyperthyroidism in undifferentiated patients with palpitations. In a large study of patients with new-onset atrial fibrillation, less than 1% were caused by overt hyperthyroidism. Therefore, although serum TSH is typically measured in the inpatient and primary care settings in patients with new-onset atrial fibrillation or palpitations, this association is uncommon in the absence of additional symptoms and signs of hyperthyroidism. Thus it is not a specific recommendation in the emergency department setting without other signs or symptoms of hyperthyroidism. 6
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