In hyperthyroidism, is Thyroid Stimulating Hormone (TSH) low and Triiodothyronine (T3) elevated?

Thyroid Function in Hyperthyroidism

In hyperthyroidism, TSH is low and T3 is elevated. 1 This characteristic laboratory pattern is a hallmark of the condition and serves as the biochemical basis for diagnosis.

Diagnostic Laboratory Findings in Hyperthyroidism

• Overt hyperthyroidism is defined by suppressed TSH and elevated concentrations of T3 and/or free T4 2
• Subclinical hyperthyroidism is characterized by low TSH with normal T3 and free T4 levels 2
• The serum TSH level in hyperthyroidism is typically below 0.1 mIU/L, with the normal reference range being 0.45-4.5 mIU/L 3
• T3 (triiodothyronine) is the peripherally active form of thyroid hormone and affects nearly every tissue and system in the body 1

Types of Hyperthyroidism

• The most common cause of hyperthyroidism is Graves' disease (approximately 70% of cases), followed by toxic nodular goiter (16%) 4
• Other causes include subacute granulomatous thyroiditis (3%) and medication-induced hyperthyroidism (9%) 4
• Central hyperthyroidism is a rare condition where thyrotoxicosis results from primary overproduction of TSH by the pituitary gland, characterized by elevated T3/T4 with non-suppressed TSH 5

Clinical Significance

• Untreated hyperthyroidism can lead to serious complications including cardiac arrhythmias, heart failure, osteoporosis, and adverse pregnancy outcomes 2
• The most prominent effects of hyperthyroidism are related to the cardiovascular system 1
• Common symptoms include anxiety, insomnia, palpitations, unintentional weight loss, diarrhea, and heat intolerance 2

Diagnostic Approach

• Confirm laboratory findings with repeat testing to rule out transient abnormalities or laboratory error 6
• Distinguish between overt and subclinical hyperthyroidism, as management strategies differ 2
• When hyperthyroidism is confirmed biochemically, further testing (TSH-receptor antibodies, thyroid peroxidase antibodies, thyroid ultrasonography, and scintigraphy) can help determine the specific cause 4

Important Caveats

• Several conditions can cause low TSH without true hyperthyroidism, including pregnancy, nonthyroidal illness (euthyroid sick syndrome), and medications like dopamine and glucocorticoids 3
• In nonthyroidal illness with low TSH, free T4 is typically in the lower part of the normal range, in contrast to the high-normal free T4 seen in typical subclinical hyperthyroidism 3
• Approximately 1-2% of patients with TSH below 0.1 mIU/L progress to overt hyperthyroidism annually, while those with TSH between 0.1-0.45 mIU/L rarely progress 3

Management Considerations

• Treatment options for overt hyperthyroidism include antithyroid drugs, radioactive iodine ablation, and surgery 2
• Beta-blockers can be used for symptomatic relief in mild or asymptomatic hyperthyroidism 6
• Close monitoring of thyroid function every 2-3 weeks is necessary during treatment to detect potential transition to hypothyroidism, which is common in thyroiditis 6

Remember that the biochemical pattern of low TSH and elevated T3 is essential for diagnosing hyperthyroidism, and this pattern helps distinguish it from other thyroid disorders like hypothyroidism, which presents with elevated TSH and low T4 levels 7.