Thyroid Function Test is the Most Important Initial Investigation
The most important initial investigation for this patient is a thyroid function test (Option B). This 36-year-old woman presents with a classic triad of hyperthyroidism: unintentional weight loss despite preserved appetite, menstrual irregularities (scanty and irregular menses), and tachycardia (pulse rate 100 bpm) with borderline elevated temperature 1, 2.
Clinical Reasoning for Thyroid Testing Priority
Key Clinical Features Pointing to Thyroid Dysfunction
- Weight loss without appetite loss is highly characteristic of hyperthyroidism, where increased metabolic rate causes weight loss despite normal or increased food intake 3
- Menstrual irregularities (oligomenorrhea with scanty flow) are a well-established manifestation of thyroid dysfunction, as thyroid hormones directly affect the hypothalamic-pituitary-ovarian axis 1, 2
- Tachycardia at rest (100 bpm) combined with the above symptoms strongly suggests a hypermetabolic state 3
- Mild temperature elevation (37.3°C) may reflect increased metabolic activity 3
Why Thyroid Testing Takes Priority
The American College of Obstetricians and Gynecologists explicitly recommends measuring TSH as essential to exclude thyroid dysfunction as a reversible cause of amenorrhea and menstrual irregularities 1. The Endocrine Society similarly emphasizes that thyroid function tests are necessary to exclude thyroid dysfunction that may present with menstrual symptoms 2. In the context of unexplained weight loss with preserved appetite, thyroid testing becomes even more critical as this combination is pathognomonic for hyperthyroidism 3.
Why Other Options Are Less Appropriate
Full Blood Count (Option C) - Secondary Priority
While a full blood count could identify anemia from heavy menstrual bleeding, this patient has scanty menses, making significant blood loss unlikely 2. Weight loss in the absence of appetite loss makes nutritional anemia less probable 4.
Faecal Occult Blood (Option A) - Not Indicated
The patient explicitly reports no altered bowel habit, and gastrointestinal blood loss would typically cause weight loss with appetite loss due to chronic disease, not the preserved appetite seen here 3. Her previous normal endoscopy further reduces concern for GI pathology.
Serum CEA (Option D) - Inappropriate for Initial Workup
CEA is a tumor marker with poor sensitivity and specificity for screening purposes 5. In a 36-year-old woman with a clinical picture consistent with a common endocrine disorder, cancer screening with CEA would be premature and not cost-effective 3. Malignancy causing this presentation would typically produce constitutional symptoms beyond what is described.
Diagnostic Algorithm Following Thyroid Testing
If TSH is suppressed with elevated free T4/T3: Diagnose hyperthyroidism and proceed with thyroid antibodies (TSH receptor antibodies for Graves' disease), thyroid ultrasound, and possible radioiodine uptake scan 3.
If TSH is elevated with low free T4: Diagnose hypothyroidism, though this would not explain the weight loss with preserved appetite 3.
If thyroid function is normal: Then proceed to evaluate other causes of menstrual irregularity by measuring prolactin, FSH, LH, and considering pelvic ultrasound 1, 2. At that point, a full blood count would be reasonable to assess for other systemic causes.
Critical Clinical Pitfall to Avoid
Never assume menstrual irregularity is benign without proper evaluation, as prolonged hypoestrogenism or undiagnosed thyroid disease increases risks of osteoporosis and cardiovascular complications 1. However, in this case, the constellation of symptoms points overwhelmingly toward thyroid dysfunction rather than primary reproductive pathology, making thyroid testing the clear first step 2, 3.