Thyroid function tests help clinicians assess the hypothalamic-pituitary-thyroid axis and identify common disorders such as primary hypothyroidism, Graves disease, thyroiditis, and central hypothyroidism. This cheat sheet gives students a fast way to match TSH, free T4, and free T3 patterns with likely diagnoses. It is useful for reviewing endocrine physiology, clinical reasoning, and lab interpretation before exams or clinical rotations.
The most important starting point is that TSH usually moves opposite to thyroid hormone levels because of negative feedback. High TSH with low free T4 suggests primary hypothyroidism, while low TSH with high free T4 or free T3 suggests hyperthyroidism. Antibody tests, medication history, pregnancy status, and illness severity help explain borderline or unusual patterns.
Key Facts
- Primary hypothyroidism usually shows high TSH and low free T4 because the thyroid gland is underproducing hormone despite pituitary stimulation.
- Subclinical hypothyroidism usually shows high TSH and normal free T4, so treatment decisions depend on symptoms, TSH level, pregnancy status, and thyroid peroxidase antibodies.
- Primary hyperthyroidism usually shows low TSH and high free T4, high free T3, or both because excess thyroid hormone suppresses pituitary TSH release.
- Subclinical hyperthyroidism usually shows low TSH and normal free T4 and free T3, and it may increase risk for atrial fibrillation and bone loss.
- Central hypothyroidism usually shows low or inappropriately normal TSH with low free T4 because the pituitary or hypothalamus is not providing adequate stimulation.
- T3 toxicosis can show low TSH, normal free T4, and high free T3, especially in early Graves disease or toxic nodular thyroid disease.
- Thyroid peroxidase antibodies support autoimmune thyroiditis, while TSH receptor antibodies support Graves disease.
- Non-thyroidal illness can cause low T3, low or normal TSH, and sometimes low free T4, so thyroid labs should be interpreted cautiously during acute illness.
Vocabulary
- TSH
- Thyroid-stimulating hormone is a pituitary hormone that increases thyroid hormone production and usually rises when thyroid hormone levels are low.
- Free T4
- Free thyroxine is the unbound circulating form of T4 and is a key measure of thyroid hormone output.
- Free T3
- Free triiodothyronine is the active unbound thyroid hormone and may be elevated in some hyperthyroid states even when free T4 is normal.
- Negative feedback
- Negative feedback is the control system in which high thyroid hormone suppresses TSH and low thyroid hormone increases TSH.
- Thyroid peroxidase antibody
- Thyroid peroxidase antibody is an autoimmune marker commonly associated with Hashimoto thyroiditis and risk of hypothyroidism.
- TSH receptor antibody
- TSH receptor antibody is an autoimmune marker that can stimulate the thyroid gland and supports a diagnosis of Graves disease.
Common Mistakes to Avoid
- Interpreting TSH alone, which is wrong because free T4 and sometimes free T3 are needed to classify overt, subclinical, and central disorders.
- Calling low TSH hyperthyroidism without checking free T4 and free T3, which is wrong because low TSH can occur in subclinical disease, non-thyroidal illness, pregnancy, or medication effects.
- Missing central hypothyroidism when TSH is normal, which is wrong because an inappropriately normal TSH with low free T4 can still indicate pituitary or hypothalamic disease.
- Diagnosing Hashimoto thyroiditis from antibodies alone, which is wrong because positive thyroid peroxidase antibodies show autoimmune risk but thyroid function depends on TSH and free T4.
- Repeating thyroid labs too soon after a medication change, which is wrong because TSH often needs about 6 to 8 weeks to reach a new steady state.
Practice Questions
- 1 A patient has TSH 18 mIU/L and free T4 0.5 ng/dL. What thyroid pattern is most likely?
- 2 A patient has TSH 0.02 mIU/L, free T4 1.3 ng/dL, and free T3 6.1 pg/mL. What pattern does this suggest?
- 3 A patient taking levothyroxine has TSH 0.08 mIU/L and free T4 1.9 ng/dL. What does this suggest about the dose?
- 4 Why can a normal TSH fail to rule out hypothyroidism in a patient with suspected pituitary disease?