Hypothyroidism: NP Diagnosis, Levothyroxine Prescribing & Monitoring

Clinical meaning

Hypothyroidism results from insufficient production of thyroid hormones (T4 and T3) by the thyroid gland, affecting virtually every organ system. The hypothalamic-pituitary-thyroid (HPT) axis regulates thyroid hormone production: the hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH). TSH binds to TSH receptors on thyroid follicular cells, activating the sodium-iodide symporter (NIS) to trap iodide, which is then organified by thyroid peroxidase (TPO) and coupled to thyroglobulin to form T4 (thyroxine, ~80% of production) and T3 (triiodothyronine, ~20%). T4 is a prohormone converted to active T3 by type 1 and type 2 deiodinases in peripheral tissues (liver, kidneys, muscle). T3 binds nuclear thyroid hormone receptors (TR-α and TR-β) to regulate gene transcription controlling basal metabolic rate, thermogenesis, protein synthesis, lipid metabolism, cardiac contractility, and CNS development. In primary hypothyroidism (95% of cases), the thyroid gland itself is impaired — most commonly from Hashimoto thyroiditis (chronic lymphocytic thyroiditis), an autoimmune condition where anti-TPO antibodies and anti-thyroglobulin antibodies mediate lymphocytic infiltration and progressive destruction of thyroid follicular cells, leading to fibrosis and goiter formation (initially) followed by atrophy. Without adequate thyroid hormone: (1) BMR decreases 30–40% causing fatigue, cold intolerance, weight gain; (2) decreased cardiac output and stroke volume with bradycardia (reduced β1-adrenergic receptor expression); (3) elevated LDL cholesterol (reduced hepatic LDL receptor expression); (4) decreased GI motility causing constipation; (5) fluid accumulation in tissues as mucopolysaccharides (glycosaminoglycans) — causing non-pitting myxedema (pretibial, periorbital); (6) impaired renal free water clearance causing dilutional hyponatremia (decreased GFR and impaired ADH suppression). Secondary hypothyroidism (pituitary TSH deficiency) and tertiary hypothyroidism (hypothalamic TRH deficiency) are rare but critical to distinguish because isolated TSH interpretation is misleading — TSH may be low or normal despite inadequate thyroid hormone levels.

Diagnosis & workup

Diagnostics & workup: - TSH (most sensitive initial test for primary hypothyroidism): elevated TSH (> 4.5–5.0 mIU/L depending on assay) is the earliest biochemical change — TSH rises before free T4 falls due to the log-linear relationship between TSH and free T4 (small decreases in T4 cause exponential increases in TSH). Subclinical hypothyroidism: TSH 4.5–10 with normal free T4; overt hypothyroidism: TSH elevated with low free T4 - Free T4 (fT4): confirms overt hypothyroidism when low; normal free T4 with elevated TSH = subclinical hypothyroidism; always measure fT4 with TSH to distinguish subclinical from overt disease - Free T3: NOT routinely measured for hypothyroidism diagnosis — T3 is maintained by compensatory upregulation of type 2 deiodinase even in moderate hypothyroidism; only useful for monitoring patients on T3-containing medications (liothyronine, desiccated thyroid) - Anti-TPO antibodies: present in 90–95% of Hashimoto thyroiditis; confirm autoimmune etiology; predict progression from subclinical to overt hypothyroidism (annual conversion rate 2–5% per year with positive antibodies vs < 1% without); useful for determining whether to treat subclinical hypothyroidism - Anti-thyroglobulin antibodies: present in 60–80% of Hashimoto thyroiditis; less specific than anti-TPO; can interfere with thyroglobulin assays (important in thyroid cancer monitoring, not relevant to hypothyroidism management) - Lipid panel: elevated total cholesterol and LDL (reduced hepatic LDL receptor expression); may see elevated triglycerides; lipid abnormalities often improve with adequate thyroid hormone replacement — check lipids 6–8 weeks after achieving euthyroidism before starting statins - CBC: normocytic or macrocytic anemia (reduced erythropoietin production, possible concurrent pernicious anemia from autoimmune polyendocrine syndrome); check B12 and folate if macrocytic - BMP: hyponatremia (impaired free water clearance from reduced GFR and impaired ADH suppression); elevated creatinine (reduced GFR); elevated CK (myopathy, rhabdomyolysis in severe cases) - Special populations — Pregnancy: TSH targets differ by trimester (1st trimester < 2.5, 2nd trimester < 3.0, 3rd trimester < 3.5 mIU/L using trimester-specific reference ranges); Central hypothyroidism: TSH may be low, normal, or slightly elevated — free T4 is the diagnostic marker; must test pituitary function (cortisol, gonadotropins, prolactin, IGF-1)

Risk factors: - Hashimoto thyroiditis (chronic autoimmune thyroiditis): most common cause in iodine-sufficient areas; anti-TPO antibodies present in 90–95% of cases; female predominance 7:1; associated with other autoimmune conditions (type 1 diabetes, pernicious anemia, vitiligo, celiac disease, Addison disease — autoimmune polyendocrine syndromes) - Post-ablative: radioactive iodine (I-131) therapy for Graves disease or toxic nodular goiter (hypothyroidism develops in 50–80% within 1 year); post-thyroidectomy (total = permanent hypothyroidism, subtotal = risk varies) - Medications: amiodarone (contains 37% iodine — can cause either hypothyroidism via Wolff-Chaikoff effect or hyperthyroidism via Jod-Basedow effect; monitor TSH q6 months), lithium (inhibits thyroid hormone release; 20–40% develop hypothyroidism), tyrosine kinase inhibitors (sunitinib, sorafenib), immune checkpoint inhibitors (pembrolizumab, nivolumab — thyroiditis in 5–10% of patients), interferon-alpha - Iodine deficiency: most common cause WORLDWIDE (rare in US/Canada due to iodized salt); causes goitrous hypothyroidism with compensatory thyroid enlargement - Subacute thyroiditis (de Quervain's): post-viral granulomatous thyroiditis causing transient thyrotoxicosis (stored hormone release) followed by hypothyroid phase (4–6 weeks) then recovery in most (80%); painful, tender thyroid with elevated ESR - Postpartum thyroiditis: occurs in 5–10% of pregnancies (higher in women with pre-existing anti-TPO antibodies); biphasic: thyrotoxicosis (1–4 months postpartum) then hypothyroidism (4–8 months) then recovery (80%) — 20% develop permanent hypothyroidism - External beam radiation to the head/neck for cancer (Hodgkin lymphoma, head and neck cancers): progressive thyroid failure over years; lifelong TSH monitoring recommended - Central hypothyroidism: pituitary adenoma, pituitary surgery/radiation, Sheehan syndrome (postpartum pituitary necrosis), infiltrative diseases (sarcoidosis, hemochromatosis, histiocytosis)

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