What thyroid med and dose are you taking? What symptoms do you have, if any? Please post your latest thyroid related test results and reference ranges shown on the lab report.
Do you take your Erfa all in one dose, or split into two doses for morning and early afternoon? Did you take your Erfa in the morning before blood draw? What time of day was the blood draw?
What about any symptoms you have?
Also, what was your ferritin level before and after supplementing? What amount of iron do you take daily?
The reason I asked about dose timing is that even the ATA/AACE Guidelines for Hypothyroidism recommend delaying the morning dose until after blood draw, in order to avoid false high results. This assumes a blood draw in the AM. I doubt that your morning dose had any significant effect on test results from a blood draw at 7 PM.
Ferritin is a significant contributors to conversion of T4 to T3. So It is possible that taking the iron supplement caused your Free T3 level to increase, resulting in the symptoms you listed. So a dose reduction was directionally correct, since symptoms are the most important consideration in making decisions about thyroid med and dosage.
Your ferritin level is good. How are your Vitamin D and B12 levels?
"Iron deficiency itself can affect thyroid function even in the absence of iodine deficiency. Studies in humans have shown that moderate-to-severe iron deficiency significantly lowers both T3 and T4 (although T3 to a greater extent) and reduces TSH responsiveness (Zimmermann, 2006). This is thought to be due to impaired thyroid peroxidase activity (Hess et al., 2002) and deiodinase activity, and responds to iron replacement.."*
*Interpretation of Thyroid Function Tests and Their Relationship to Iodine Nutrition - C.M. Dayan, V. Panicker, in Comprehensive Handbook of Iodine, 2009
Your Vitamin D and B12 should be supplemented to optimize. For best effect D should be at least 50 mcg/mL and B12 in the upper part of the range.
The effect you mention from optimizing ferritin is usually not an issue because most hypothyroid patients are dosed based on TSH, which doesn't work for the patient. As a result of inadequate med/dosage due to dosing based on TSH, the patients usually continue having lingering hypo symptoms, so the ferritin effect would not be noticed.
Instead of dosing a hypothyroid patient based on TSH, it should be titrated as needed to eliminate the signs/symptoms of hypothyroidism, without creating any signs/symptoms of hyperthyroidism. Note the following conclusion from a recent excellent scientific paper that quantified for the first time the effect of Free T3 level on hypo symptoms. "Hypothyroid symptom relief was associated with both a T4 dose giving TSH-suppression below the lower reference limit and FT3 elevated further into the upper half of its reference range."