Thyroid Hormones 101 for Primary Care NPs

A quick refresher on the thyroid axis and the most common causes of hypo- and hyperthyroidism you’ll see in clinic.

The Thyroid Axis (TRH → TSH → T4/T3)

• Hypothalamus releases TRH → stimulates the pituitary.

• Pituitary releases TSH → stimulates the thyroid to produce T4 (thyroxine) and T3 (triiodothyronine).

• Negative feedback:

  • Small increases in serum T4/T3 → decrease TSH secretion.

  • Decreases in T4/T3 → increase TSH secretion.

Clinical pearl: In primary hypothyroidism, TSH rises before T4 falls, making TSH a sensitive early marker.

Common Causes of Hypothyroidism

• Hashimoto’s thyroiditis (chronic autoimmune)

  • Most common in iodine-sufficient regions

  • Cell- and antibody-mediated destruction; TPO antibodies positive

  • May be goitrous or atrophic

• Post-thyroidectomy

• Radioiodine therapy or external neck irradiation

• Drug-induced (e.g., amiodarone, lithium, interferon; check the med list)

Common Causes of Hyperthyroidism

• Autoimmune: Graves’ disease (diffuse goiter, ophthalmopathy) or Hashitoxicosis (transient hyperthyroid phase of Hashimoto’s)

• Thyroiditis (painless/postpartum, subacute) — often transient

• Toxic nodules: single toxic adenoma or multinodular goiter

• TSH-secreting pituitary adenoma (rare; inappropriately normal/high TSH with high T4/T3)

• Excess thyroid hormone (over-replacement or surreptitious use)

Bedside Pearls

• TSH ≠ one-and-done: When abnormal, add free T4 (and sometimes total/free T3) to define the pattern.

• Antibodies help the “why”:

  • Suspected Hashimoto’s → TPO Ab

  • Suspected Graves’ → TRAb/TSI

• Thyroiditis clues: Tender thyroid (subacute), recent pregnancy (postpartum), low uptake on uptake scan (if done).

• Medications matter: Amiodarone and lithium can cause hypo or hyper; always review meds.

• Don’t miss central causes: If TSH isn’t suppressed despite high T4/T3, think pituitary (TSHoma, assay interference).

What’s Next

Next week we’ll dive into interpreting thyroid labs in primary care:

• How to approach high TSH/low T4 vs low TSH/high T4/T3

• When to order antibodies, uptake scans, or ultrasound

• Treatment starting points and follow-up intervals

Education only; use clinical judgment and local guidelines.

Questions or tricky cases? [email protected]  or DM me on social.
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