Thyroid Hormones 101 for Primary Care NPs

A TSH lands in your inbox and you need to know what it's really telling you. Before you can read the labs, it helps to have the basic wiring straight in your head.

Here's the quick version of the thyroid axis and the causes of hypo- and hyperthyroidism you'll actually see in clinic.

How does the thyroid axis work?

Three glands talk to each other in a loop:

• The hypothalamus releases TRH, which signals the pituitary.
• The pituitary releases TSH, which tells the thyroid to make T4 (thyroxine) and T3 (triiodothyronine).
• Negative feedback keeps it balanced. When T4 and T3 go up, TSH drops. When T4 and T3 fall, TSH climbs.

That feedback loop is why TSH is your first-line test. In primary hypothyroidism, TSH rises before T4 actually falls, so it catches a struggling thyroid early. Want the full read on labs? See how to read thyroid labs fast.

What causes hypothyroidism?

• Hashimoto's thyroiditis (chronic autoimmune). This is the most common cause in iodine-sufficient areas like the US. It's antibody-mediated destruction, TPO antibodies are usually positive, and the gland can be enlarged or shrunken.
• Post-thyroidectomy. If the gland is out, the patient needs replacement.
• Radioiodine therapy or external neck irradiation.
• Drug-induced. Amiodarone, lithium, and interferon are the usual culprits. Check the med list before you go hunting for anything exotic.

What causes hyperthyroidism?

• Graves' disease. Autoimmune, with a diffuse goiter and sometimes eye changes. This is the most common cause of true hyperthyroidism.
• Thyroiditis. Painless, postpartum, or subacute. These are often transient, so the hyperthyroid phase can resolve on its own.
• Toxic nodules. A single toxic adenoma or a toxic multinodular goiter making hormone on its own.
• Hashitoxicosis. A transient hyperthyroid phase early in Hashimoto's.
• TSH-secreting pituitary adenoma. Rare. You'll see an inappropriately normal or high TSH alongside high T4/T3.
• Excess thyroid hormone. Over-replacement or surreptitious use. Always recheck the dose.

Pearls to keep in your back pocket

• TSH alone isn't the whole story. When it's abnormal, add a free T4 (and sometimes a T3) to define the pattern before you act.
• Antibodies answer the "why." Suspected Hashimoto's, order TPO. Suspected Graves', order TRAb or TSI.
• Thyroiditis clues: a tender thyroid points to subacute, recent pregnancy points to postpartum.
• Medications cut both ways. Amiodarone and lithium can cause hypo or hyper, so always review the med list.
• Don't miss central causes. If TSH isn't suppressed despite high T4/T3, think pituitary or assay interference.

Frequently asked questions

Is TSH enough to screen for thyroid disease?
For most asymptomatic patients, a TSH is a reasonable first test. If it's abnormal or the patient has clear symptoms, add a free T4 to see the full picture.

Do I need to order antibodies on everyone?
No. Reserve antibodies for when they change your plan, like confirming Hashimoto's in new hypothyroidism or sorting out the cause of hyperthyroidism.

Which is more common, hypo or hyperthyroidism?
Hypothyroidism is far more common in primary care, and Hashimoto's drives most of it in the US.

Once you've got the axis down, the next step is reading the labs and treating. Walk through hypothyroidism treatment and hyperthyroidism first steps. If you want a one-page version at your desk, the Clinical Desk Reference covers thyroid and more for $37. And if you want the full system for working through any visit, the Primary Care Clinical Mastery Program is AANP-accredited.

Education only. Use clinical judgment and your local guidelines.

Written by Allison Sowders, MSN, APRN, FNP-BC, a practicing primary care nurse practitioner and founder of Nurse Practitioner Mentor. Reviewed July 2026.