THYROID-FUNCTION TEST

SUMMARY

HIGH TSH
1. High TSH + low free T₄ = primary hypothyroidism, Hashimoto's thyroiditis
2. High TSH + high free T₄ = secondary (pituitary) hyperthyroidism
3. High TSH + normal free T₄ = subclinical hypothyroidism (takes a lot for the pituitary to push the thyroid gland to work)

LOW TSH
4. Low TSH + low free T₄ = secondary/tertiary hypothyroidism
5. Low TSH + high free T₄ = thyrotoxicosis
6. Low TSH + normal free T₄ = sick euthyroid syndrome, nodular goitre with autonomous production

NB: if hyperthyroidism is established (low TSH & high FT₄), then perform radioactive iodine uptake (RAIU) +/- thyroid scan (scintigraphy)

Reference(s)

Wilkinson, I., Furmedge, D. and Sinharay, R. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press. Get it on Amazon.
Feather, A., Randall, D. and Waterhouse, M. (2020). Kumar And Clark’s Clinical Medicine. 10th ed. S.L.: Elsevier Health Sciences. Get it on Amazon.
Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
Therapeutic Guidelines. Melbourne: Therapeutic Guidelines Limited. https://www.tg.org.au [Accessed 2021].