12.3 Thyroid and Other Endocrine Disorders

Thyroid disease, adrenal incidentalomas, vitamin D deficiency, and PCOS occupy the rest of the endocrine system box. Domain II wants TSH-first reasoning and a Rotterdam-style PCOS workup. Domain III Planning wants a starting levothyroxine dose that will not provoke ischemia in an older adult. Domain IV Implementation is refusing casual methimazole and recognizing adrenal crisis as an EMS event.

Start with TSH

TSH is the first test in stable outpatients when you suspect hypo- or hyperthyroidism. If TSH is high, add free T4 to distinguish subclinical from overt hypothyroidism. If TSH is low, add free T4 and often total or free T3 to distinguish subclinical hyperthyroidism, overt hyperthyroidism, and the rare central (pituitary) pattern in which TSH is inappropriately normal or low with a low free T4. TPO antibodies support Hashimoto disease and help predict progression of subclinical hypothyroidism; they do not replace TSH for dosing. Do not order a full thyroid panel as a wellness reflex in an asymptomatic adult with no risk — that is how you chase noise.

Biotin in high-dose supplements can wreck thyroid assays. Stop biotin for a few days and repeat before you treat a bizarre result.

Hypothyroidism: starting doses that match the heart

Young, healthy adults with overt primary hypothyroidism can start near full replacement: about 1.6 mcg/kg/day of lean body weight. A 70-kg lean 32-year-old is not a 25-mcg project.

Older adults and anyone with known or suspected CAD start at 12.5–25 mcg daily and titrate every 6 weeks. A full 1.6 mcg/kg load can unmask angina or demand ischemia. Frail elderly stay at the low end.

Recheck TSH about 6 weeks after a dose change (levothyroxine's half-life makes four to five half-lives land near 5–6 weeks — Section 9.1). Take the tablet on an empty stomach; separate from calcium, iron, and many antacids by several hours. PPIs and celiac disease reduce absorption. Do not use levothyroxine as a weight-loss drug in a euthyroid adult. Do not treat a single TSH of 4.8 in an 84-year-old with no symptoms by jumping to 100 mcg.

Liothyronine (T3) combination therapy is not first-line primary care. If a specialist is already using it, do not stop it casually on a wellness visit without a plan.

Pregnancy in a woman already on levothyroxine: increase the dose by about 20–30% as soon as pregnancy is confirmed (a common practical instruction is two extra tablets per week) and check TSH every 4 weeks in the first half of pregnancy. Trimester-specific TSH targets are lower than the nonpregnant range — conceptually, aim toward the lower half of the trimester-specific reference (many protocols use a first-trimester upper bound near 2.5–4.0 depending on the assay). Recheck after delivery and reduce toward the prepregnancy dose. Untreated overt hypothyroidism is a miscarriage and neurodevelopmental problem, not a wait-until-the-next-annual issue.

Hyperthyroidism: refer, beta-block, do not freelance methimazole

Low TSH plus high free T4 is Graves disease, a toxic nodule or multinodular goiter, or thyroiditis until you sort the etiology. The FNP move is:

1. Confirm the labs and examine for orbitopathy, a bruit, a tender thyroid (thyroiditis), and a dominant nodule.
2. Beta-block symptomatic tachycardia, tremor, and palpitations (propranolol is traditional; a cardioselective agent is acceptable if asthma is an issue and a beta-blocker is still appropriate).
3. Refer to endocrinology for uptake/scan when needed, antithyroid-drug decision, or surgery discussion.
4. Do not start methimazole casually without a plan: you need a working diagnosis (thyroiditis is not treated with methimazole), a pregnancy test in anyone who can be pregnant (methimazole is teratogenic especially in early pregnancy; PTU is preferred in the first trimester if a drug is required), baseline CBC and liver enzymes, and counseling for agranulocytosis (fever, sore throat — stop the drug and get a same-day CBC) and hepatotoxicity.

Thyroid storm — fever, delirium, arrhythmia, high-output failure — is an ED diagnosis. Postpartum thyroiditis can flip from hyper- to hypothyroid; treat the phase you are in, not the phase you fear.

Nodules: ultrasound risk, then FNA

A palpable nodule or an incidental nodule on imaging gets a TSH and a thyroid ultrasound, not an immediate FNA of every 8-mm cyst. If TSH is low, the next step is a scan (hot nodules are rarely malignant) rather than a needle. If TSH is normal or high, use an ultrasound risk system (ACR TI-RADS or ATA pattern) plus size to decide FNA. High-suspicion solid hypoechoic nodules with irregular margins, microcalcifications, or taller-than-wide shape meet FNA at smaller sizes than spongiform or purely cystic nodules. Pure cysts and spongiform nodules under usual size cutoffs are observed. Document vocal-cord symptoms and compressive signs — those are surgical conversations, not return in a year.

Adrenal incidentaloma and adrenal crisis

An adrenal incidentaloma is an adrenal mass found on imaging done for another reason. The FNP job is twofold: is it functioning, and is it malignant-looking?

• Imaging: size ≥4 cm, heterogeneity, high attenuation, or irregular margins raise concern — refer.
• Function: screen for autonomous cortisol (1-mg overnight dexamethasone suppression is the usual first test), aldosteronism if hypertension or unexplained hypokalemia (aldosterone and plasma renin), and pheochromocytoma (plasma or urine metanephrines) especially if hypertension, spells, or the imaging looks vascular. Do not biopsy a possible pheo.
• Refer endocrinology or endocrine surgery for functioning lesions, larger lesions, or uncertain imaging.

Adrenal crisis is shock, hyponatremia, hyperkalemia, hypoglycemia, abdominal pain, and often fever in a person with known adrenal insufficiency or chronic glucocorticoids who stopped the drug or became septic. Treat now: parenteral hydrocortisone (or the glucocorticoid you have) plus volume, and send to the ED. Do not wait for an ACTH-stim test in a crashing patient. Teach stress-dose steroids and an emergency injection kit to every adult on chronic replacement.

Vitamin D and calcium: treat deficiency, do not megadose everyone

Measure 25-hydroxyvitamin D when you suspect deficiency (malabsorption, dark skin plus low sun, osteoporosis workup, CKD, hypocalcemia), not as a cash-pay wellness add-on for every tired adult. Treat documented deficiency with a repletion course (a common adult pattern is weekly high-dose cholecalciferol for 6–8 weeks then daily maintenance) and correct calcium and phosphate if they are part of the picture. Do not megadose 50,000 units weekly forever in a sufficient adult for immunity. Toxicity is hypercalcemia, polyuria, and kidney injury.

Dietary calcium first. Supplements if diet cannot meet needs, especially in osteoporosis treatment alongside vitamin D. Do not assume more calcium is always safer in older adults (stones, and no extra cardiovascular benefit from megadosing). If hypercalcemia appears, check PTH next — primary hyperparathyroidism versus PTH-independent causes (malignancy, granulomatous disease, vitamin D excess).

PCOS as endocrine–reproductive overlap

Use Rotterdam criteria conceptually: two of three after you exclude other causes — (1) oligo- or anovulation, (2) clinical or biochemical hyperandrogenism, (3) polycystic ovarian morphology on ultrasound. Exclude pregnancy, thyroid disease, hyperprolactinemia, and, when indicated, nonclassic congenital adrenal hyperplasia (17-hydroxyprogesterone). Do not diagnose PCOS from acne alone in a regularly cycling teen, and do not skip the metabolic work: OGTT or A1c, lipids, blood pressure, BMI and waist, OSA symptoms, and endometrial protection if she has long stretches without withdrawal bleeding (combined hormonal contraception or cyclic progestin). Metformin helps metabolic features and cycle regularity in some; it is not a fertility guarantee. Letrozole for ovulation induction is often specialty-directed. Screen for diabetes more aggressively than in age-matched women without PCOS.

Vignette. A 28-year-old with oligomenorrhea, BMI 31, and hirsutism needs a pregnancy test, TSH, prolactin, and a hyperandrogenism assessment before the PCOS label, plus glucose testing — not spironolactone as the first and only act. A 74-year-old with new TSH 18 and free T4 low after a NSTEMI last year starts 25 mcg, not 125 mcg. A 31-year-old on 100 mcg levothyroxine who texts a positive pregnancy test gets an immediate 20–30% dose increase and a TSH this month, not a congratulations-only reply.